Abstract: Objective The effects of mercury （Hg） and selenium （Se） on the total anti-oxidative capacity （T-AOC） of liver and gill tissues and the antagonistic effect of Se on the change of T-AOC of liver and gill in swordtail （Xiphophorus helleri Heckel） initiated by Hg were investigated in order to test if the liver or gill T-AOC change in fish can be considered as a valid biomarker for environmental risk assessment （ERA）. Methods Swordtails exposed to Hg with 0,0.21 and 0.42 mg/L, Hg plus Se with （0.21+0.0664） and （0.42+0.0664） mg/L, Se with 0.0664 mg/L using the addictive index method, the changes of T-AOC of liver and gill tissues in 5 days were determined quantitativelly. Results The T-AOC of liver and gill tissues were decreased in different level when swordtails were exposed to Hg during the experimental period；high Hg concentrations could remarkably （P＜0.01） decrease the T-AOC of gill and liver tissues to 39% and 30% of the control group at fifth day, however, the T-AOC of gill and liver tissues were significantly increased （P＜0.05） when swordtails were exposed to Se first day after exposure. Compared to the single Hg treated, the mixed exposure （low Hg concentration plus Se） could restrain the fall of the T-AOC in gill tissues all through the exposure period； however, the T-AOC in liver tissues was significantly decreased （P＜0.05） at fifth day. High Hg concentration plus Se the mixed exposure could only restrain the fall of T-AOC in gill and liver tissues at first day, and then the T-AOC of gill and liver tissues were significantly decreased （P＜0.05 or P＜0.01） as the time increased. Conclusion The Se could prevent the decrease of T-AOC in liver and gill tissues from the poisoning of Hg in fish at a certain extent.In addition, the T-AOC of gill and liver tissues in swordtails c6uld be considered a valid biomarker for environmental risk assessment （ERA）.