Electron spin resonance (ESR) method was used for studying radicals generated in low and high density polyethylene (LDPE and HDPE) by gamma ray irradiation. Decay rate of the radicals were determined as a function of storage time at different temperatures, according to which, appropriate storage time and temperature could be chosed for the irradiated polyethylene. Identification and quantitative analysis of radicals originated from different types of polyethylene was done with the PE samples irradiated under ambient or N,2, atmosphere. For the relationship between radical concentration and irradiation dose, the results showed a nearly linear increasing of R·, (relative concentration of radicals) with doses in the range of 10-100 kGy. Under the same dose, R· of the HDPE samples was higher than LDPE, and HDPE was chosen for pre-irradiation grafting. In the case of irradiation HDPE under ambient atmosphere, decay rate of R· was dominated by quantity of oxygen diffusing into the HDPE matrix.