Objective To investigate the antitumor activities of three selenium compounds in EM>vitro/EM> as well as EM>in vivo/EM>, and to explore the mechanism. Methods Assay of MTT and growth inhibition of S180 and H22 on tumorbearing mice were used to study the antitumor effect, morphological observation, flow cytometry and confocal laser scanning microscopy assay were used to study the mechanism of three selenium compounds. Results They could significantly inhibit proliferation of K562 cells in vitro（EM>P/EM><0.05）, and the growth of S180 and H22 (EM>P/EM><0.01,EM>n/EM>=10). Electron microscopic observation revealed typical apoptotic features, including shrinkage of cellular and nuclear membranes, condensed heterochromatin around the nuclear periphery, and cytoplasmic vacuolation in the K562 cells treated with NaSeVO, SeMoV and SeWV 5 mg/L for 24 hours. The cell cycle was redistributed by selenium compounds and a significantly subG showed at high dosage. The fluorescence intensity of intracellular CaSUP>2+/SUP>,MgEM>SUP>2+/SUP>/EM> and ROS was greatly increased after treatment with selenium compounds as compared with control group（EM>P/EM><0.01）. However, the fluorescence intensity of intracellular pH value and MMP decreased（EM>P/EM><0.01）. Conclusion Three selenium compounds have antitumor activity EM>in vivo/E