The novel polyoxometalates, (H2imi)2[Bi2W20O70Co2(H2O)6Na4(H2O)14].2H3O.15H2O (imi=iminazole) (1) has been synthesized and structurally characterized. The inhibitory activities to several human cancer cells were studies in this study. The cell viability of 1 on human colon HT-29 cells were determined by MTT, some cell morphology data, and single cell gel electrophoresis etc. analysis. The cell morphologic and DNA changes were confirmed that 1 could play the proliferation inhibitory effect on cancer cells. 1 induced apoptosis on HT-29 cells. The results suggested that the possible mechanism is related to DNA apoptosis in cancer cells.