A backlight module (10) includes a light source and a light guide plate (12). The light source defines a number of light units (11) for emitting light beams. The light guide plate includes a light incident surface (121), an emission surface (123) adjacent to the light incident surface, a bottom surface (124) opposite to the emission surface, a plurality of side surfaces (122) between the emission surface and the bottom surface, and a plurality of diffusion units (13) formed on the bottom surface. A dot size of each diffusion unit/dot is inversely proportional to a summation of the sum of reciprocals of squares of distances between the diffusion unit and each of the light units and the sum of reciprocals of squares of distances between the diffusion unit and corresponding images of each of the light units formed, respectively, by the side surfaces.