540 grass carps ［initial body weight （8.25±0.37） g ］ were randomly divided into 6 treatments (90 for each treatment, 30 for each replicate) to investigate the nutritional physiological effects of dietary isoleucine on grass carp by feeding with six graded levels of crystalline isoleucine (0.77 g, 1.07 g, 1.37 g, 1.67 g, 1.97 g and 2.27 g per 100 g dry diet) which have the same isonitrogenous diets (35.25 % protein) with caseingelatincorn gluten mealcrystalline amino acid mixtures as protein source supplemented. Amino acid pattern in diet is to simulate the amino acid pattern found in the whole body protein of grass carp except for isoleucine. Each diet was randomly assigned to triplicate groups of 30 juvenile grass carp in freshwater floating net cages (1.0 m ×1.0 m ×1.0 m). After a 72day feeding trail, the results showed that: When the dietary isoleucine level was 1.67 % level, weight gain (WG), specific growth rate (SGR), protein efficiency ratio (PER) got the maximum and feed conversion rate (FCR) got the minimum (P<0.05); deinternal organ ratio (DOR) and condition factor (CF) got the maximum, viserosomatic index (VIS) got the minimum; moisture and crude lipid of body were minimum, protein and ash content of it were the maximum(P<0.05); Moisture of muscle was minimum, protein were the maximum(P<0.05), but the difference of crude lipid was not significant(P<0.05). When the dietary isoleucine reached 1.67 % level, both the content of isoleucine and total amino acids in the muscle were maximum. The RNA/DNA of muscle rose first and then went down, when the dietary isoleucine reached 1.67 % level, RNA/DNA of muscle were the maximum, and the difference was significant(P<0.05). Glutamate hydrogenase activity of liver went down first and then raise, when the dietary isoleucine reached 1.67 % level, glutamate dehydrogenase activity was the lowest, and the difference was significant(P<0.05). When the content of isoleucine went up, the blood ammonia was leveling off at the beginning, when the dietary isoleucine reached 1.67% level, the blood ammonia rose. The difference of the albumin in the serum was not significant(P<0.05). TG and CHO went down first and then rose, when the dietary isoleucine level was 1.67 % level, TG and CHO were the lowest, and the difference was significant among groups(P<0.05). The conic analysis of WG, SGR, FE and PER against dietary isoleucine level indicated that optimal dietary isoleucine requirement for juvenile grass carp was 1.46 %,1.49 %,1.45 %and 1.41 % of the diet (corresponding to 4.14 %, 4.23 %, 4.11 % and 4.0 % of dietary protein on a dry weight basis). So the range of optimal dietary isoleucine requirement for juvenile grass carp was 1.41%-1.49%(dietary base) or 4.0 %-4.23 %(protein base).