An exemplary backlight module (300) includes a light source (320), a frame (310) and diffusing plate (340). The frame includes an inner bottom surface (314) and an inner side surface (312) adjacent to the bottom surface. The side surface includes a light reflecting and converging structure. The diffusing plate cooperates with the frame to define a space that accommodates the light source. The side surfaces having the v-shape structures can reflect and converge light beams toward the middle portion of the diffusing plate, thus preventing these light beams from otherwise obliquely entering peripheries of the diffusing plate and subsequently being lost. Therefore, the backlight module has an increased light utilization ratio.