A back side illumination (BSI) image sensor is provided. The BSI image sensor includes a semiconductor substrate, a first dielectric layer, a reflective element, a second dielectric layer and a color filter layer. The semiconductor substrate has a front side and a back side. The first dielectric layer is disposed on the front side of the semiconductor substrate. The reflective element is disposed on the first dielectric layer, in which the reflective element has an inner sidewall contacting the first dielectric layer, and the inner sidewall has a zigzag profile. The second dielectric layer is disposed on the first dielectric layer and the reflective element. The color filter layer is disposed on the backside of the semiconductor substrate.