Abstract: In this study, we investigated the effects of dietary methionine (Met) on the growth performance, the serum index, the activities of key enzymes in Met metabolism, and the oxidation of juvenile groupers (Epinephelus coioides). Six iso-nitrogen and iso-lipid diets were prepared with DL-Met supplement at different concentrations, 0.71%, 0.98%, 1.26%, 1.57%, 1.86%, and 2.18% (Diet1-Diet6). Each treatment was randomly assigned to a triplicate of 30 fish initial weight (9.750.05) g per aquarium. Fish were fed at 8: 00 and 17: 00 every day and were maintained in a flow- through aquaria for eight weeks. The results showed that the weight gain and the specific growth rate of the Diet3 group were significantly higher than those of the Diet1 and Diet6 groups (P0.05). The condition factor of the Diet4 group was significantly higher than that of the Diet1, Diet5 and Diet6 groups (P0.05). The levels of serum total proteins of the Diet2 and Diet3 groups were significantly higher than that of the Diet5 group (P0.05). The level of blood glucose of the Diet3 group was significantly lower than that of the Diet1 and Diet2 groups (P0.05). The level of serum total cholesterol was gradually reduced in the Diet3 group, and it was significantly decreased in the Diet4 and Diet6 groups compared to the Diet2 group (P0.05). The activities of superoxide dismutase (SOD) and catalase (CAT) in the liver of the Diet3 group were the highest (P0.05). There was no significant difference in the activity of phosphoenolpyruvate carboxylase kinase (PEPCK) between the Diet3 and the Diet4 groups, but it was significantly lower than that in the other groups (P0.05). Quadratic regression analysis of the specific growth rate corresponding to the level of dietary methionine indicated that the optimal concentration of dietary methionine for the growth of juvenile groupers was 1.42% of dry diet in the presence of 0.46% cystine (corresponding to 3.16% of dietary protein on a dry weight basis). At this level, the blood glucose, the serum total cholesterol and the activity of PEPCK could be relatively low, which would help improve the energy efficiency in fish metabolism. Also the increased activities of SOD and CAT may help protect the body from oxidative damages.