A controlled/living radical polymerization, initiated by γ-radiation and followed by a post-polymerization process, of methyl methacrylate (MMA) was carried out in the presence of 2-cyanoprop-2-yl 1-dithionaphthalate. The polymerization showed first-order kinetics. The molecular weights of the corresponding polymers increased linearly with conversion. The molecular weight distributions ( M w / M n ) of the polymers decreased with the conversion (minimal M w / M n value: 1.09). The polymers were characterized by 1H NMR and gel-permeation chromatograph. Chain-extension reaction was also successfully carried out to obtain higher molecular weight PMMA with narrow molecular weight distribution.