A backlight module includes a light guide plate and only one light source. The light guide plate has a main body and protrusions. The main body has a light emitting surface, a light incident surface, a first side surface, and a second side surface. The protrusions are positioned on the light incident surface, and face the light source. The protrusions diffuse the light rays from the only one light source and guides the light rays into the main body. The light emitting surface has a light emitting area and a marginal area surrounding the light emitting area. The marginal area has a light diffusion area adjacent to the light incident surface and the first side surface. An included angle between the light incident surface and the first side surface is an obtuse angle. An included angle between the light incident surface and the second side surface is an obtuse angle.