Polyoxometalates (POMs) are negatively charged clusters consisting of transition metals and oxygen atoms. There are two general types of POM activity, antiviral and antitumoral, have dominated the medicinal chemistry of these compounds. Based on synthesized four different kind of POMs, [Ce(H2O)7Cr(OH)6Mo6O18]n·4nH2O, [Sm(H2O)7Cr(OH)6Mo6O18]n·4nH2O, Eu(H2O)7Cr(OH)6- Mo6O18]n·4nH2O, and K6[P2W18O62]·14H2O, we investigate their antitumor effect in vitro. K6[P2W18O62]·14H2O exerted the most strong activity. IC50 of it for MCF-7, SKOV-3 and HepG2 were 22μmol/L, 19μmol/L and 24nmol/L respectively. Lower concentration of K6[P2W18O62]·14H2O could induce HepG2 cells apoptosis. It up-regulated the expression of Bax and Bim, apoptosis promoting members in Bcl-2 family, so the mechanism of this effect may through mitochondria path way.