A backlight unit including an LED module including a plurality of LEDs emitting light, and an LED substrate supporting and driving the plurality of LEDs, a diffusion sheet placed over the LED module and diffusing light emitted from the LED module, an optical sheet placed over the diffusion sheet, a module supporter placed on a back of the LED module and supporting the LED module, a chassis assembly supporting the module supporter and the diffusion sheet, and a plurality of support pins supporting the diffusion sheet and the optical sheet against the chassis assembly. Thus, a backlight unit having an improved structure in assembling efficiency is provided.