A light sensing illumination device for bicycle includes a seat, a reflector, and an illumination device. The seat includes a housing space. The reflector is disposed in the seat and in the housing space, and the reflector includes a reflection curved face which is formed in a smooth arc shape with a concave face thereof facing an opening of the housing space. The illumination unit is disposed in the housing space and projects light beam toward the reflection curved face. With a smooth arc shaped reflection curved face, the light beam projected by the illumination unit is completely reflected, so as to improve the light condensing effect and efficiently enhance the overall brightness.