A back light module includes a light guide plate, at least one first light source, at least one second light source and at least one first reflection element. The light guide plate includes a first light incident surface, a second light incident surface opposite to the first light incident surface, a light emitting surface, and a bottom surface opposite to the light emitting surface. The first light source is disposed beside the first light incident surface and suitable to provide a first non-collimated light beam to the first light incident surface. The second light source is disposed beside the second light incident surface and suitable to provide a first collimated light beam to the second light incident surface. The first reflection element is disposed beside the first light incident surface to reflect the first collimated light beam emitted so as to make the first collimated light beam diverge.