A backlight unit capable of implementing slimness and preventing stain defects is disclosed. The backlight unit includes a plurality of LEDs arranged in a fixed interval, a reflection sheet disposed under the LEDs and configured to reflect light, a diffusion plate disposed on the LEDs and configured to diffuse light, and a plurality of inversed pyramid patterns formed along a diagonal direction in the center of one side of the diffusion plate on the upper surface of the diffusion plate. The plural LEDs include a plurality of odd-numbered line LEDs and a plurality of even-numbered line LEDs arranged in zigzags.