Four new vanadium substituted Dawson-type heteropoly acids H[PMoVO]·39HO(1), H8[P2Mo16V2O62]·41H2O(2) H9[P2Mo15V3o62]·51H2O(3)and H8[P2Mo14V4O62H2]·45H2O(4)were prepared respectively. Their structures were determined by IR and ICE. The inhibition effects of vanadium substituted Dawson-type heteropoly acids(Mo, P) on free radical polymerization of methyl methacrylate(MMA) were investigated by dilato- metry. The results show that the rate of the polymerization of MMA decreases and the inhibition effects of the four heteropoly acids reach the inhibitor performance of hydroquinone at a certain ratio.