A backlight unit capable of preventing a light leakage phenomenon caused by using a light guide plate of wedge type is disclosed. The backlight unit includes: at least one light source; a light guide plate configured to include a first portion with a first thickness disposed adjacent to the at least one light source, a second portion of plate shape having a second thickness thinner than the first thickness, and a wedge portion formed to connect the first and second portions and become gradually thinner from the first portion to the second portion; and a flexible printed circuit board loaded with the at least one light source and extended to cover the wedge portion of the light guide plate. The flexible printed circuit board is provided with a groove formed opposite a boundary area between the first portion and the wedge portion.