In this paper, the influence of light induced electrons on the threshold voltage shift of organic thin film transistors (OTFTs) was studied. The light induced electrons are formed and accumulated in the channel near the interface of gate dielectric. The light source can he used as second source. Therefore, the study demonstrates the light induced electrons in channel can be affected by drain bias like bias stress. The decreasing accumulated electrons with increasing positive drain bias lower the threshold voltage shift rate during illumination time. The opposite trend can be observed when using negative drain increases accumulated electrons. This result is helpful to adjust the photosensivity of organic photo transistors.