Provided are an image sensor using a light-sensitive oxide semiconductor material as a light-sensitive device and a method of operating the image sensor for acquiring RGB values of incident light in the image sensor, the image sensor includes an array of a plurality of light-sensing cells wherein each of the light-sensing cells includes a light-sensitive oxide semiconductor layer that forms a channel region of an oxide semiconductor transistor. Electronic characteristics of the light-sensitive oxide semiconductor layer vary according to an amount of light irradiated onto the light-sensitive oxide semiconductor layer. Each of the light-sensing cells constitutes a single unit color pixel.