A CMOS image sensor is provided. The CMOS image sensor includes a photodiode receiving light and generating photocharges, a transfer transistor connected to the photodiode and transferring the photocharges, a floating diffusion accumulating the photocharges transferred from the transfer transistor, a reset transistor discharging the photocharges accumulated in the floating diffusion, and a merge gate transistor controlling capacitance of the floating diffusion. The CMOS image sensor may obtain a wide dynamic range signal without an increase in size of a pixel.