Abstract: The tilapia (GIFT Oreochromis niloticus) with initial body mass of (74.36±5.32) g were fed by diets at six myo-inositol (MI) levels (0 mg·kg^–1 , 100 mg·kg^–1, 200 mg·kg^–1, 400 mg·kg^–1, 800 mg·kg^–1 and 1 600 mg·kg^–1 ) diets for 12 weeks, so as to investigate the effect of MI on tilapia. We measured their growth performance, body composition and serum biochemical indices, and finally determined the requirement of dietary MI for large size GIFT O.niloticus. The results show that the dietary MI increased the weight gain rate (WGR), feed intake and feed conversion ratio significantly compared with the control group (P<0.05). According to the second-order polynomial regression analyses based on WGR and dietary MI level, the optimum MI requirement was 847 mg·kg^–1. The crude fat contents in the liver, muscle and whole body of tilapia decreased with increasing dietary MI level, but the crude protein contents in muscle and liver showed no significant difference among groups (P>0.05). The activities of aspartate aminotransferase and alanine aminotransferase in serum were significantly lower in 800 mg·kg^–1 and 1 600 mg·kg^–1 MI groups than those in the control (P<0.05). With increasing dietary MI level, the total cholesterol and triacylglyceroln contents in serum firstly deceased and then increased, but the glucose contents in serum showed an opposite trend. In conclusion, the diet with 847 mg·kg^–1 MI is optimal for large size GIFT O.niloticus, and appropriate dietary MI supplementation is beneficial to reducing tissue fat accumulation and improving liver function.