A unit pixel of an image sensor is provided. The unit pixel includes a photoelectric conversion element configured to generate photocharge varying with the intensity of incident light, a transfer transistor configured to transfer the photocharge to a floating diffusion in response to a transfer control signal, and a supplemental transistor connected to the floating diffusion. Because the unit pixel includes only one transistor in addition to the transfer transistor, the area of the unit pixel is minimized, and, as a result, the resolution of a pixel array is increased and the power consumption of the pixel array is decreased.