A backlight module has a light guide plate, a back plate and light emitting assemblies. The light guide plate is substantially prism-like. The back plate has a shape corresponding to that of the light guide plate, and has an inclined first side plate and an inclined second side plate opposite to the first side plate. Each of the first and second side plates has at least one receiving recess, and the light emitting assemblies are received in the receiving recesses. In comparison with a side-light type backlight module, the present invention saves a heat-dissipation aluminum extrusion member. In comparison with a direct-light type backlight module, the present invention shortens the light mixing distance in a vertical direction, so as to reduce the module thickness.