The polymerization of methyl methacrylate mechanochemically initiated by the grinding of several inorganic compounds was attempted using a vibrating ball mill. The inorganic compounds used were quartz, quartz glass, marble, limestone, feldspar, and talc with the particle size range of 149-210 mum. The results demonstrated that all compounds used in these experiments had the activity for mechanochemical polymerization of methyl methacrylate, and that the degree of the activity at the same grinding time remarkably differed with the kind of compound. The existence of the induction period, which is the time until the polymerization started, was also confirmed in all of the compounds. The molecular weight distributions of the polymer formed were unimodal and somewhat broad in the case of any compound, and the number-average molecular weight was similar to that formed by ordinary radical polymerization. It was suggested that there were two types of the initiation mechanism in the mechanochemical polymerization of methyl methacrylate initiated by the grinding of inorganic compounds.