A backlight module (20) includes a light guide plate (22), a light source (21), and a reflection plate (23). The light guide plate includes a light incidence surface (221) for receiving light, a light emitting surface (223) for emitting light, and a bottom surface (222). The light source is disposed adjacent the light incidence surface. The reflection plate disposed under the bottom surface includes a base (232), and a reflection layer (233) formed on the base. The reflection layer defines a number of diffraction grating units (231) at an outer surface thereof. Grating constants of the diffraction grating units progressively decrease with increasing distance away from the light incidence surface. This enables the light emitting surface to output highly uniform light. Further, if the process of fabrication of the diffraction grating units fails, only the reflection plate need be discarded. Thus the backlight module has a low mass manufacturing cost.