In this work, we have investigated the effect of gate voltage on the performance of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methylester (PCBM) bulk hetero-junction based organic phototransistor. Transistor was fabricated in MESFET configuration with top gate and bottom drain-source electrodes on glass substrate. Active layer showed Schottky type contact with Gate electrode and Ohmic contact with Drain-Source electrodes. Current-Voltage (1-V) characteristics of the device were studied under dark and UV -Vis illumination. Active layer of the device has shown p-type and am bipolar properties under dark and UV-Vis illumination, respectively. Drain to source current of the phototransistor was found dependent on the illumination intensity and gate to source voltage. Photo sensitivity and responsivity values of the device were found to decrease exponentially with the increase of gate voltage. Photo sensitivity and responsivity values of the device were found equal to 11 and 0.024 AIW, respectively, at 90mW/ cm2 UV-Vis illumination intensity and 0 gate voltage.