Four experiment diets were designed (purified diet , practical diet and practical diet with lysine uncoated or coated ) . Grass carp (mean weight of 7. 27 ±0. 5g) were fed separately in 12 fiberglass tanks (100cm ×50cm ×50cm) with 30 fish per tank. Every experiment diet was taken three repeats. The experiments lasted 9 weeks. Results indicated that grass carp fed with the purified diet showed a significantly higher growth ratio and protein synthesis rate ( Ks) than fish fed with the practical diet ( P < 0. 05) . Practical diet with coated lysineimproved the growth rate, feed conversion, balance of essential amino acid in plasma (BEAAp ) and protein synthesis rate ( Ks) in comparison with diet with lysine uncoated or deficiency. Coated lysine , but not free lysine , proved to be a significantly better way of dietary protein supplementation.