The electron spin resonance (ESR) technique was introduced to study the photocarrier generation mechanism in the dual-layered organic photoreceptors containing DEH in the carrier transport layer (CTL) and bisazo pigments synthesized by coupling 1,5-diaminoanthraquinone with various 2-hydroxy-3-anphthanlidies in the carrier generation layer (CGL). The results showed that the photocarrier efficiency strongly depended upon the spin concentration of the bisazo pigments under irradiation. The dependence was successfully elucidated by the reaction rates of photocarrier generation elementary processes.