A pixel unit of an image sensor is provided. The pixel unit includes a semiconductor substrate, a light-sensitive element, a contact and a protection layer. The contact is formed right on the light-sensitive element to enable electrical signals outputted from the light-sensitive element to be transmitted to a peripheral circuit. The protection layer is disposed on the light-sensitive element and surrounds the first contact. The electrical signals of the light-sensitive element can be upward transmitted to the peripheral circuit through the contact. Therefore, the light-sensitive element can occupy a big area, and high quantum efficiency (QE) is achieved accordingly.