Juvenile Japanese f lounders were fed with purif ied diets containing different levels of eicosapentaenoic acid ( 20: 5n- 3; EPA) and docosahexaenoic acid ( 22: 6n- 3,DHA) to determine the effects of the both fatty acids on fish growth performance. Combining the growth data with the analysis of fatty acid composition of muscle and liver, it was indicated that the optimum requirement for EPA and DHA are 0. 5% and 1. 0% - 1. 5% , respectively. Under the conditions, the f ish fed with diet 5 and diet 6 showed highest growth rate, lowest muscle moisture and HI, higher muscle lipid content and highest EPA, DHA accumulation in the polar lipid fraction of the liver. However, in the non-po lar lipid fract ion of the liver and muscle, the compositions of fatty acid showed no signif icant difference between the groups. So the contents of polar lipid fraction could indicate the ef fects of n- 3 HUFA on fatty acid composition of f ish muscle, and EPA, DHA contents in the polar fraction were much higher than those of non-polar fraction. Either in liver or in muscle, the contents of 16: 0 and 18: 1n- 9 were quite high both in polar and non-polar lipid fraction. So the ratio of 18: 1n- 9 and n-s HUFA could determine if the diet EFA contents could meet the requirements. The ratio will reduce when fish grow s well, in the present experiment, the f ish fed diet 5 and diet 6 showed lowest 18: 1n- 9/ n- 3HUFA, they were 0. 62 and 0. 74, respect ively.