Three countries; Korea, China and Japan have tried to participate in various multinational Free Trade Agreements while they push for a Free Trade Agreement(FTA) among them. In this situation, it remains to be seen whether it is necessary for three countries to promote their own close economic cooperation at practical level. Until now, there are many studies on the structure of intra-trade among them and the method of their economic cooperations including FTA. Those studies have mainly focused on possibilities of three countries' economic cooperations through comprehensive research at industrial level. But this paper studied the structure of intra-trade among them through analysing trading items on the base of HS Code Heading(4 digit) and tried to reason out necessities of three countries' economic cooperations. Through analysis of three countries’ intra-trade volume, several results are drawn as follows. Firstly, it is inferred that there are necessities of economic cooperations among them through the analysis of three countries' economic positions in the world economy. Secondly, it is reasoned out the necessity of economic cooperation of two countries(Korea and Japan) with China through analysis of changes of other country's shares in each country's total trade volume. Thirdly, it is inferred that there are weaknesses in three countries' intra-trade structure through analysis of export- import items' concentrativeness and duplication. In order to remedy these weaknesses, it is reasoned out that it is necessary for three countries to make close economic cooperation. Lastly, through analysis of trade specialization index on export-import items of three countries' intra-trade, it is inferred that there is relativity of competitiveness among them and Korea has more necessity of economic cooperation than China and Japan. In conclusion, it is shown that there are always necessities of economic cooperation among three countries and that for Korea, close economic cooperation with China and Japan are very important.