This study aimed to investigate lexical diversity of advanced Korean learners by analyzing their writings. Advanced learners are composed of foreign learners and heritage learners and Korean native speakers were employed to collect their writings as comparison group. To meet the goal, the study conducted two different genre-writing tasks and analyzed the writings by number of tokens, and number of types. TTR was implemented to examine lexical diversity and lexical diversity of meaning classes in the writings of three groups. The study found out that (i) there was a significant difference in number of tokens and number of types among three groups, (ii) both number of tokens and number of types showed significant difference according to groups and writing genres, (iii) no significant relation was found in lexical diversity among groups, but (iv) each group showed a significantly different lexical diversity between the genres. As for results from lexical diversity analysis of meaning classes, heritage learners showed no difference with native speakers in using of concrete and clear vocabulary, time and emotional expression vocabulary, but significant difference in using of professional and detailed vocabulary, intention and value, and abstract vocabulary; however, foreign learners showed significant difference in all meaning classes with heritage learners