Here we report the effect of gate voltage on the performance of organic phototransistors made using bulk heterojunction films of regioregular poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C(61) (PCBM). To understand the illumination effect, a monochromatic light (520 nm) corresponded to the maximum absorption of present P3HT: PCBM blend film was employed. Results showed that the present device followed a p-type field-effect transistor in the dark and under illumination though the saturation trend of output curves became weakened under illumination. The responsivity exhibited a two-stage trend as a function of gate voltage. The maximum responsivity reached 0.13 A/W under illumination of green light at 81 mu W/cm(2).