To determine dietary leucine(Leu)requirement of tilapia(GIFT Oreochromis niloticus),tilapias with an initial body weight(53.65±0.05)g were randomly divided into seven treatments in triplicates with 14 fishes per repetition.Seven isonitrogenous(crude protein 32%) and isoenergetic semi-purified diets containing different levels(1.11%,1.51%,1.90%,2.29%,2.69%,3.08% and 3.48%) of dietary Leu were fed to tilpias at the feeding rate of 4%-6% for 56 d. Quadratic regression analysis showed that the optimal dietary Leu levels of tilapia were 2.28%-2.33% base on weight gain rate(WGR),specific growth rate(SGR),feed conversion ratio(FCR). With an increase of dietary Leu level,the crude protein and crude lipid content of whole body and muscle initially increased and then decreased, but there were no significant differences in moisture content of muscle.With an increase of dietary Leu level,activities of intestinal amylase activity,intestinal protease ,Na⁺-K⁺-ATPase initially increasing and then decreased,and the lipase activity get maximum at 2.69% level and the others got maximum at 2.29% level.With the increase of dietary Leu level,the concentrations of EGF and EGFR,and the activities of SOD,LZM and AKP also initially increased and then decreased,all got maximum at 2.29% level.In conclusion,the optimal dietary Leu level for 50-200 g GIFT tilapia was 2.28%-2.33%(or 7.11%-7.26% of dietary protein).This Leu level can significantly promote the development of intestinal mucosa,enhance gastro-intestinal digestion and absorption capacity,thus improve the body's nonspecific immunity,and then promote the growth and utilization of tilapia.