In this study, we discuss the toxicity of Hg 2+ , Se 4+ to Carassius aurats and the antagonistic effects of Se 4+ . The influence of Hg 2+ , Se 4+ on the level of lipid peroxidation (the content of malondialdehyde, MDA) and the total antioxidant capacity (T-AOC) of liver cells were measured in liver cells and the viability of liver cells, the permeability of liver cells, the membrane fluidity of liver mitochondria, the mitochondrial membrane potential (Δψm) were monitored using several fluorescence indicators. The results showed that the levels of MDA, the T-AOC activity, the viability of liver cells, the permeability of liver cells, the fluidity of cell membrane, the Δψm, significantly induced by Hg 2+ (2 or 10 μM, 12 or 64 h), were inhibited to a different extent, respectively, when cells were pretreated with sodium selenite (2 or 10 μM, 12 or 64 h) after exposure to Hg 2+ , reducing the level of MDA, increasing the activity of T-AOC, enhancing the viability of liver cells, increasing the membrane fluidity of liver mitochondria, and improving the mitochondria transmembrane potential. In conclusion, selenium supplement as a micronutrient could maintain the antioxidant capacity of liver cells, decrease the generation of MDA induced by Hg 2+ exposure, and consequently protect liver cells partly from damage by stabilizing the membranes of cell and organelles such as mitochondria.