Abstract: To explore the combined effects of body mass and water temperature on the oxygen consumption and ammonia excretion rate of starved wild mullets (Mugil cephalus) in the South China Sea, we applied two-factor five-level central composite design and response surface method on the wild mullets (5.66−27.68) g captured near Daya Bay under different experimental water temperatures (16−32 ℃). The results show that the smaller the body mass and the higher water temperature are, the more significant effect of oxygen consumption rate and ammonia excretion rate are, and body mass has significantly better influence on oxygen consumption and ammonia excretion rate than water temperature. Under different conditions, the n(O):n(N) ratio varied from 10.15 to 19.52. When the water temperature was 24 ℃ and the body mass was (27.68±0.38) g, the n(O):n(N) ratio was the highest, which reveals that the metabolicism of protein and fat is vigorous and has a fast growth rate. Therefore, it is determined that the optimum growth temperature for mullets is 24 ℃.