Recently, demands of high resolution printing by electrophotography are increased. To realize high resolution, high gamma characteristics of surface voltage decay versus irradiation energy are paid attention to. High gamma characteristics can make the electrostatic image formed on the photoreceptor sharp compared with the light image irradiated on the photoreceptor. The mechanism of high gamma characteristics is considered due to structural trap. On the base of the structural trap model, surface voltage decay is calculated by changing the characteristic length of the structural trap. It is obtained, when the characteristic length increase, the gamma value of surface voltage decay increases. The gamma values obtained by experiments are compared with the calculated results and the characteristic length is estimated.