The Koreans in the last period of the Koryo Dynasty(918-1392) subordinated under the Mongol empire were forced to have lived for a century long with the small number of two foreign elements, namely Mongols and Uighurs, whose cultures and languages were much different. In the period the influences of the dominant Mongols were clearly recorded and accordingly remained clear even now, but those of the Uighurs who accompanied the Mongols as assistants of the administration to the Korean peninsular comparatively left their presences dimly behind in the history of Korea, not easy to find out. Most of the Uighurs were also Muslims, but some of them werenot. As a result, there were some confusions for the Koreans to clearly distinguish the Uighurs from the Muslims. In general, the editors of the official history book, entitled Koryosa(高麗史) mentioned distinctly the two terms; the Uighur and the Muslims. However, some statements of the book are sometimes obscure, especially in the designation of some Muslims(回回), who were not surely known to be the Uighur. For example the Muslim A-sil-mi-ri(回回 阿實迷里) at the article of the leaf month of March, 1276(King Chungryol 2)and Kim-bi-hoe-hoe(金鼻回回) at the article of November, 1387(King Shinwoo 13). The racial origin of the former was not clearly mentioned in the book and the other sources. That of the latter was certainly Korean, but recorded as a Muslim. Recently a plenty of the Korean modern translations of the classical works in the period including the Koryosa have been published with notes. And lots of secondary books authored by the modern scholars also followed up. The interpretations by the translators and the explanations by the scholars usually regard the Uighurs as the Muslims and vice versa without showing any further proofs. There are the two figures, namely Chang Sunryong(張舜龍) and Seol Son(偰遜), representing respectively the Muslims and the Uighurs in the period. The forefathers of the former was originated from the Muslim majority region of China, namely Sozhou(韶州) of the province Guangzhou(廣州), south to the lake Dongting(洞庭湖), which do not belong to the Uighur majority area. He, therefore, is surely regarded as Muslim, but not as Uighur. But it is impossible to prove it because of lack of materials available on his forefathers. The Chinese home of the latter’s forefathers is Gouchang(高昌), Sinzang(新疆省) of China, once the capital region of Kara Khoja Buddhist Kingdom(860-1339). According to the history of his family, his fifth forefather(五代祖) left the region to take a post under the Mongol empire, long before the fall of the kingdom from the Muslim west. It is, thus, certain that he is not a Muslim, but a Uighur. Both of them, Chang Sunryong(張舜龍) and Seol Son(偰遜), were, later on, naturalized in the Dynasty. The meaning of the naturalization at that time was quite different from that of the modern days. That was just to change the typical style of the original personal name in their respective home country into that of the adopted country, while this meant to change the nationality of one country into that of another country. In detail, the former was to change his name of Samga(三哥) into Chang Sunryong, while the latter did not change his name written in Chinese characters at all because the Korean type of name was exactly the same as the Chinese one. Most of the naturalized Koreans seemed to put on the clothes of their old country or Korea as they will, just after the change of their names. But the Muslims had attached to their traditional clothes, even after their naturalization in Korea. They were, later on, forced to wear the Korean ones according to the edict of King Sejong(1418-1450). Most of the prominent personalities in the Period such as kings, ministers had usually two kind of names, Korean and Mongolian. But it is unknown that the naturalized Korean Muslims of the foreign origins used to have the two kind of names, Korean and Islamic. The case of Ramadhan(刺馬丹) on his official post as Darugachi(達魯花赤) who died and buried near and in Guangzhou(廣州), China, suggests us that he must have the two names. Unfortunately his Korean name is not known now. Instead of it his Islamic name is still on his gravestone. The name has proved that there surprisingly existed some Korean converts to Islam, who were slowly disappeared among the mass together with the Uighur Muslims in the flow of time. The two most striking features of the Uighur influences on the Korean culture in the period were; the Uighur letters adopted from the Sogds and Islam. The letters based on the sound systems were usually written by the Mongolian princesses to their parents in Beijing who got married to the kings of the Dynasty and later on, learnt by Korean kings and ministers. They had some impacts on the Koreans, who had the unshakable concepts on the Chinese characters which had enjoyed the status of the official languages since the period of Three Kingdoms(三國時代). They also became an impetus to invent the Korean letters by King Sejong and his ministers who grew up under such social atmospheres. Islam believed by the majority of Uighurs at the time was a total failure in the viewpoint of its propagation to the Koreans gaining only fewer converts, but successful to some extent in spreading its name among the mass of the Koreans because the name of Islam was mentioned even in one of the folkpoems of Korea named Ssangwhajom(高麗歌謠 雙花店). In the other hand a figure of a Muslim, known as Hoehoeseja(回回世子, literally meaning Muslim crown prince) with the personal name Munhae(文偕) was awkwardly combined with the Korean traditional beliefs(서낭신앙). In conclusion the cultural exchange between two heterogeneous cultures like both Korean and Uighur can be productive in initiating their respective inventive ideas and pursue them until achieving the successful results like invention of the Korean letters. But the combination of two unequaled partners like the case of a Muslim with a kind of the Korean traditional beliefs can come out as a byproduct of abnormal meeting of the two.