An image sensor includes a pixel that includes a photoelectric conversion element converting an incident light to an electrical signal, a switch adjusting a capacitance of a floating diffusion (FD) node at which charges corresponding to the electrical signal are stored, and a readout circuit outputting an output voltage based on the FD node. An A/D converter may sample the output voltage transferred from the readout circuit through an output line respectively at a first time and a second time and generate a digital code based on a difference therebetween. A conversion gain controller may generate a conversion gain control signal by comparing the output voltage transferred from the readout circuit through the output line with a threshold voltage at a third time between the first and second times and provide the conversion gain control signal to the switch to set conversion gain of the pixel.