A backlight module includes a light source and a light guide plate. The light source includes a light-emitting surface and at least two mean points. The light guide plate includes an incident surface, an emitting surface adjacent to the incident surface, a reflecting surface opposite to the emitting surface, and at least two groups of microstructures formed on the reflecting surface. Each group of microstructures is arranged concentric relative to a respective mean point of the light-emitting surface of the light source.