Poly(methacrylonitrile) (PMAN) samples in the shape of a cylinder used in this study were obtained from methacrylonitrile by bulk polymerization. The electron spin resonance (ESR) method has been used to calculate oxygen diffusion coefficient (D) into PMAN samples exposed to high-energy radiation at different doses in vacuum by observing the ESR radical signal change. In order to calculate the dose-independent diffusion coefficient (D 0 ), ln D values were plotted against dose values. The low dose region of this curve was extrapolated to a zero-dose value, and D 0 was calculated as D 0 Ο 3.1 Ο« 10 Οͺ9 cm 2 /s. Although D 0 values were in very good agreement with the one obtained for the thin-film sample of PMAN, the dose dependence of the oxygen diffusion into the cylindrical PMAN samples was observed as being converse of the thin-film of PMAN, 1 as expected, because of the big difference of the surface/mass values between relatively big cylindrical PMAN samples and thin-film of PMAN samples. The activation energy (E a ) values of the oxygen diffusion into PMAN were calculated as E a1 Ο 27.9 kJ/mol for the 20 -60Β°C temperature range E a2 Ο 74.2 kJ/mol for the temperatures above 60Β°C of the 36 kGy gamma-irradiated samples. The temperature value of the break point of activation energy was near to the T g of PMAN. 2