This paper is concerned with the conduction electron concentration in an insulator exposed to high energy radiation. We find that the steady state conductivity [sgrave] (∞) of insulators is proportional to the dose rate of irraliation if the radiation source is weak and the temperature is low; the [sgrave] (∞) could be proportional to square root of the dose rate if temperature is very high. Before the steady state is reached, The approximate solution of electron concentration in conduction band of an insulator is calculated to change exponentially with time.