Abstract: In the conventional 4T-APS pixel circuit of CMOS image sensor the threshold voltage of source follower varies with the soure voltage, and that results in the nonlinearity of the input-output characteristic, which will have a severe effect on the image quality.This paper proposes a linearization method for pixel circuit which corrects the nolineartity of the source follower in the pixel circuit without effectting the fill factor of the pixel.The simulation suggests that the output error of the conventional source follower is 205.9 mV, while the output error of the proposed one is no more than 300 μV.