A human eye has the logarithmic response over wide range of light intensity. Although the gain can be set high to identify details in darker area on the image, this results in saturation in brighter area. The gamma correction is essential to fit the human eye. However, the digital gamma correction degrades image quality especially for darker area on the image due to the limited ADC resolution and the dynamic range. This paper proposes a CMOS image sensor (CIS) with nonlinear analog-to-digital converter (ADC) which performs analog gamma correction. The CIS with the proposed nonlinear ADC conversion scheme was fabricated with a 0.35-μm CMOS process. The test results show the improved image quality than digital gamma correction.