The invention discloses an optical sensor. Some embodiments of the present disclosure provide an optical sensor. The optical sensor includes a semiconductive block having a front side and a back side, a wave guide region, and a light sensing region. The wave guide region is positioned over the back side of the semiconductive block and having a core layer. The wave guide region is configured to guide an incident light. The light sensing region is positioned in the semiconductive block, having a multi-junction photodiode. The light sensing region is configured to sense emission lights from the wave guide region. According to the invention, the photodiode is arranged between a biological sample and an interconnect, and the optical element is separated from an electric element of an image sensor. A metal interconnect opens the light from the sample, so that light loss is reduced, and a high quantum efficiency is achieved. With the light sensing region approaching the sample, the light sensing region is exposed to even more intense light to be tested.