Abstract: 【Objective】To investigate the pharmacokinetics and changes of sulfadiazine and trimethoprim in the blood of Oreochromis niloticus at different water temperatures.【Method】Compound sulfadiazine which contained two composition of sulfadiazine and trimethoprim was orally administered to O. niloticus at a single dosage of 120 mg/kg at 18,23,28 and 33 ℃.The concentration of sulfadiazine and trimethoprim in O. niloticus plasma was determined by HPLC method.【Result and conclusion】Results showed that higher peak plasma concentration(C_max) of sulfadiazine was 12.41,19.60, 22.48 and 30.78 μg/mL at 18, 23, 28 and 33 ℃，the C_max of trimethoprim was 1.22, 2.06, 2.44 and 2.70 μg/mL respectively，and a higher peak plasma concentration was found at a higher temperature. The elimination(t_{1/2 ke}) of sulfadiazine was 18.22, 17.89, 16.90 and 12.99 h at 18, 23, 28 and 33 ℃；t_{1/2 ke} of trimethoprim was 16.39, 7.08, 5.99 and 4.04 h respectively. Fast elimination of trimethoprim and sulfadiazine was observed at a higher temperature in O. niloticus. Concentration ratios of sulfadiazine/trimethoprim were 9.57∶1-11.01∶1, 6.30∶1-9.36∶1,5.40∶1-10.39∶1 and 4.20∶1-20.64∶1 within 10 h after administration at the synergistic activity ratios ranging of 1∶1-40∶1. The results show that the temperature can significantly affect the absorption and elimination of the drugs in O. niloticus, improving the maximum plasma concentration. It also speeds up the elimination of the drugs, but has little effect on the ratio of compound sulfadiazine.