Isothermal annealing of color centers in gamma ray irradiated (100 kGy) polystyrene in vacuum and air atmospheres is investigated. The color centers were measured by the optical absorptions at wavelength 400 nm. The absorption is assumed to be linearly proportional to the concentration of color center. The color center annihilation follows the second-order kinetic process for the vacuum annealing and the first-order kinetic process for the air annealing. Both rate constants of annealing in vacuum and air satisfy the Arrhenius plot. The activation energy is 54.29 kJ/mol for the former and 56.62 kJ/mol for the latter.