A backlight module including a light guide plate and a light source is provided. The light guide plate has a light incident surface, a light reflection surface, a first side surface and a second side surface. The light incident surface is connected to the second side surface. The reflection surface is connected between the first side surface and the light incident surface. The first and the second side surfaces are two planes with their extending planes intersected. The light incident surface is a chamfering plane connected between the first and the second side surfaces. The light reflection surface is a cambered surface connected between the first side surface and the light incident surface. The light source is disposed next to the light incident surface, so as to transmit light into the light guide plate through the light incident surface.