In order to investigate the effects of lysine and dissolved oxygen on grass carp, the grass carp were fed with 13, 15 and 17 g kg −1 lysine diet at about 6 mg L−1 (high dissolved oxygen, HO group) or 3.5 mg L−1 (low dissolved oxygen, LO group) dissolved oxygen level, for 8 weeks. The fish were fed to apparent satiation by hand. The results showed that apparent digestibility of protein, energy and dry matter were decreased significantly when grass carp were fed at 3.5 mg L−1 dissolved oxygen, and feed intake (FI) was also inhibited by low dissolved oxygen (P < 0.05). Weight gain, protein retention, protein efficiency, feed conversion ratio and amino acid retention of fish at 6 mg L−1 dissolved oxygen level were significantly improved at 3.5 mg L−1 dissolved oxygen level (P < 0.05). Weight gain, protein and amino acid retention, and feed efficiency of grass carp at the two dissolved oxygen levels were significantly improved by lysine supplementation (P < 0.05). The dietary lysine level and dissolved oxygen of water had an interaction effect on feed conversion ratios (P < 0.05). Grass carp fed at low dissolved oxygen level showed lower liver protein and fat contents. Plasma aspartate aminotransferase (AST) activity of grass carp fed at 3.5 mg L−1 dissolved oxygen level was significantly increased compared to 6 mg L−1 dissolved oxygen level (P < 0.05). Our results show that low dissolved oxygen level of water is harmful to the liver of grass carp.