A backlight module includes a light guide plate and a light source. The light guide plate includes a first end portion, a second end portion, a middle portion between the first and second end portions, a top face extending from the first end portion to the second end portion, a bottom face extending from the first end portion to the second end portion, a light entrance face interconnecting the top and bottom faces at the first end portion, and a plurality of prismatic strips formed on the bottom face. The prismatic strips have a lower density at the first end portion as compared to the middle portion. The density is defined as a total length of the prismatic strips per unit area of the bottom face. The light source is disposed adjacent to the light entrance face.