A 10-week feeding trial was conducted to investigate the dietary lysine requirement of blunt snout bream (Megalobrama amblycephala) with initial body weight of 52.49±0.18 g (size Ⅰ) and 101.85±1.85 g (size Ⅱ). Six isonitrogenous and isoenergetic semi-purified diets were formulated that containing graded lysine levels (1.29%, 1.71%, 2.09%, 2.48%, 2.88% and 3.27%of dry weight). The results showed thatweight gain rate (WGR), specific growth rate (SGR) and protein efficiency ratio (PER) in fish of size Ⅰ fed with diets containing 2.88% and 3.27% lysine were significantly higher than that of the control group, while the reverse pattern was observed in feed conversion ratio (FCR) and feed intake (FI). The 2.48% lysine diet significantly increased SGR and decreased FCR, meanwhile, 3.27% lysine diet significantly increased SGR and FI in fish of size Ⅱ.The diet with 2.88% and 3.27% lysine significantly increased body fat content in fish of size Ⅰ and size Ⅱ, respectively.Significantly decreased ALT in plasma of size Ⅰ and Ⅱand increased Urea of size Ⅰ were observed fed with 2.48% lysine diet, while increased ALB, TP and GLB in plasma content of size Ⅱ fed with 3.27% lysine diet, which indicate that proper lysine in diet might protect the liver and enhance the metabolism and immunity ability compare with the control group. The 2.48%, 2.88% and 3.27% lysine diet significantly increased the plasma lysine content of size Ⅰ and Ⅱ, and decreased the plasma arginine and total essential amino acids content of size Ⅰ. Based on the broken-line regression analysis of SGR, the optimal dietary lysine requirements of size Ⅰ and Ⅱ were estimated to be 2.07% and 2.19% (6.27% and 6.63% of dietary protein), respectively.