With increasing market demand of high resolution CMOS(Complementary Metal Oxide Semiconductor) image sensors, its unit pixel size has dramatically de-creased over the years. Therefore, Number of pho-tons entering unit pixel on an integration time have been decreased as unit pixel size decreases inevitably resulting in lower sensitivity of each pixel. Various technologies to increase the sensitivity have been researched and applied in recent image sensors with small sized pixel. In this paper, we introduce optimization method for higher sensitivity in CMOS image sensor and inevitable side ef-fects are successfully reduced.