The mechanism of trap effect in the process of photoconductivity of photorefractive materials is proposed in this paper. The kinetic equations of carrierls light stimulating, recombination, and trapping is presented, and the theoretical expression of the quantum efficiency of photogeneration with trapping effect is obtained. In experiment, the dependence of the quantum efficiency of PVK thin film doped C60(C70)of different concentration has been measured, and the result is. good agreement with the proposed theory.