In this work, atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) was successfully carried out at room temperature (25 degrees C) under Co-60 gamma-irradiation environment. The polymerization proceeded smoothly with high conversion (>90%) within 7 h. The polymerizations kept the features of controlled radical polymerization: first-order kinetics, well-predetermined number-average molecular weights (M-n,(GPC)), and narrow molecular weight distributions (M-w/M-n < 1.25). H-1 NMR spectroscope and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry confirmed that poly(methyl methacrylate) (PMMA) chain was end-capped by the initiator moieties. The Cu(II) concentration could reduce to 20 ppm level while keeping good control over molecular weights. This is the first successful example for the ATRP of MMA under Co-60 gamma-irradiation at room temperature. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem49: 3588-3594, 2011