The polymerization behavior of zinc methacrylate in rubber (HNBR and NBR)/zinc methacrylate/peroxide compound was investigated by measuring the quantity of unreacted Zn(MAA)2 and grafted poly-Zn(MAA)2 and the molecular weight of poly-Zn(MAA)2 and the tensile strength of the compound.The conversion of unreacted Zn(MAA)2 to poly-Zn(MAA)2 depends mainly on the peroxide concentration and little on the rubber. The conversion increases with the peroxide concentration. However, the peroxide concentration affects the conversion little at more than 90% conversion. The number of grafted poly-Zn(MAA)2 on NBR inversely relates to ln[uncured Zn(MAA)2] and is 4.6 times as high as that of on HNBR at 80% conversion. In the case of HNBR, the number slightly increases up to 80% conversion and sharply increases above 80%. The molecular weights of poly-Zn(MAA)2 is inversely proportional to [peroxide]1/2 at low peroxide concentration, however, is little dependent on the peroxide concentration at high peroxide concentration and high conversion. The molecular weight on NBR compound is about one fourth of that on HNBR compound. Considering the higher number of grafted poly-Zn(MAA)2, the molecular weight of poly-Zn(MAA)2 on NBR compound is defined by radical transfer to the rubber. The tensile strength of HNBR compound is higher than 50MPa above 80% conversion. These results show that Zn(MAA)2 polymerizes in solubirized state, not in solid state.