The hardness of irradiated hydroxyethyl methacrylate (HEMA) copolymer at elevated temperatures was measured using a microhardness tester. The hardness increases with annealing time, and is attributed to the defects present in the molecular chains. The defects that control the hardness are related to the entanglement of polymer chain and follow a first order kinetics process. The relaxation time satisfies the Arrhenius equation, with constant activation energy of 25 kJ/mol independent of the irradiation dose. The results were compared with those of PMMA and LiF single crystals reported in the literature. The present findings are useful in the study of soft contact lens, kidney dialysis system, drug delivery system, and artificial liver support system. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 657-661, 2009