An exemplary backlight module (1) includes a light guide plate (10) and a light source (12) arranged to provide light beams to the light guide plate. The light guide plate includes a light emitting surface (102), a bottom surface (102) opposite to the light emitting surface, and a plurality of reflective structures (104) at the bottom surface. The reflective structures substantially define a plurality of isosceles triangular profiles, and each of the reflective structures maintains a pair of base angles relative to the bottom surface. The base angles of each reflective structure are the same, the base angles of all the reflective structures are the same, and the common base angle is in the range from 45.degree. to 55.degree..