Radiation-induced polymerization of methyl methacrylate (MMA) was studied up to 7500 kg/cm2at 20°C. The rate of polymerization increased to 3000 kg/cm2with overall activation volume ΔVpol‡ of -23.6 cm3/mole, and then the pressure dependence of the rate was very small in the pressure range between 3000 and 3700 kg/cm2. The rate of polymerization increased again above 3700 kg/cm2up to the crystallization pressure of MMA (5500 kg/cm2) with ΔVpol‡ of -13.7 cm3/ mole with increasing pressure. The volume contraction by polymerization decreased with increasing pressure up to 3000 kg/cm2but hardly decreased with increasing pressure above 3000 kg/cm2. The stereoregulzarity (triad probability) of PMMA changed slightly at 3000 kg/cm; above 3000 kg/cm2, syndiotactic addition decreased and heterotactic addition increased. Marked change in P-V isotherms of MMA, however, was not observed about 3000 kg/cm2. We concluded from these facts that an alignment of monomer molecules, which does not cause large volume change, was realized about 3000 kg/cm2. Polymerization proceeded above the crystallization pressure by long time irradiation, and isotactic addition increased clearly in the solid-state polymerization.