A photodiode for an image sensor capable of reducing reflection of light incident onto the photodiode and effectively absorbing transmitted light and a method of manufacturing the same are provided. In the photodiode for the image sensor, a silicon concavo-convex surface with a nano-thickness is formed by forming silicon oxide (SiO, x=0.5-1.5) layer on a silicon substrate and treating the silicon oxide layer with heat. A photodiode region is formed under the silicon layer having convexes and concaves. In this case, light absorptance increases because light reflected on the silicon concavo-convex surface is reincident onto another convex or concave. Therefore, an effective depth of the photodiode is larger than that of a planar photodiode, and accordingly, quantum efficiency of the photodiode increases.