A back light unit includes a light source and a light guide unit having a first surface emitting light to a display panel, a second surface facing the first surface, and a plurality of connecting surfaces connecting the first surface and the second surface. The connecting surfaces includes a first side surface, a second side surfaces and a third side surface between the first side surface and a second side surface, A first included angle of the first incident surface and the first diffusion surface is equal to or greater than about 90 degree.