Effect of irradiation temperature on the main-chain scission of poly (methyl methacrylate) (PMMA) caused by gamma-irradiation was studied by means of gel permeation chromatography and ESR spectroscopy. Although no temperature dependency was observed on the scission efficiency for purified PMMA, the efficiency for crude or monomer-doped purified PMMA was decreased by decreasing the temperature below ca. 200 K. Above 200 K the efficiency was constant and did not depend on the purity of PMMA. ESR study of the irradiated PMMA revealed that the suppression of the scission below 200 K is induced by the addition of methyl methacrylate monomer to primary radical species, which otherwise cause the main-chain scission by warming the polymer above 200 K. The primary radical generated above 200 K immediately converts to the scission-type -CH2-C(CH3)COOCH3 radical through the p-scission of the polymer main chain, so that the efficiency of the scission does not depend on both the impurity and the irradiation temperature.