Seven isonitrogenous and isolipidic commercial diets with arginine contents of 2.13%, 2.42%, 2.71%, 2.95%, 3.20%, 3.48%, or 3.74%(dry matter) were formulated to investigate the effects of dietary arginine on growth performance, feed utilization, and serum total nitric oxide synthase in the juvenile orange-spotted grouper, . Triplicate groups of 30 fish with mean body weight of (7.52±0.02) g were randomly assigned to each treatment in separate tanks. The fish were fed the experimental diets for 8 weeks. The different levels of dietary arginine had no significant effect on the fish survival rate ( > 0.05). However, the weight gain rate and specific growth rate were significantly higher in the 2.95% arginine group than in the other groups ( < 0.05). The feed coefficient of the fish fed 2.95% arginine was significantly lower than that of the fish fed 2.13%, 2.42%, or 2.71% arginine ( < 0.05). The whole-body protein of the fish fed 3.20% arginine was significantly higher than that of the fish fed 2.13% arginine ( < 0.05). Serum glucose tended to decrease as dietary arginine increased, and was significantly higher in the fish fed 2.13% or 2.42% arginine than in those of the other groups ( < 0.05). Total protein and total nitric oxide synthase were significantly higher in the sera of fish fed 2.95% arginine than those in the sera of fish in the other groups ( < 0.05). These results indicate that dietary arginine not only promotes growth and body protein synthesis, but also improves the immunity of the grouper. A quadratic regression analysis of the specific growth rate against dietary arginine indicated that the optimum arginine level for the maximum growth of the juvenile grouper is 3.06%, corresponding to 6.07% of dietary protein on a dry weight basis.