An exemplary backlight module (1) includes a light source (11) and a light guide plate (12). The light guide plate includes a light incident surface (121) to which the light source is placed adjacent, and a bottom surface (122) adjacent the light incident surface. The bottom surface defines a plurality of recesses (1222). Each recess defines a first slant (101) facing the light source and an arris (103) at a top of the first slant. The arris of the first slant at each recess is aligned substantially perpendicular to a main path of light beams that propagate from the light source to the recess.