A backlight unit is provided. The backlight unit includes: a first surface light source array; a second surface light source array disposed on an upper portion of the first surface light source array and arranged such that a light emitting surface of the second surface light source array is parallel with a light emitting surface of the first surface light source array; and a reflection plate disposed on a lower portion of the first surface light source array and arranged such that a reflective surface of the reflection plate is parallel with the light emitting surface of the first surface light source array, wherein the second surface light source array is stacked on the first surface light source array such that the first and second surface light source arrays are offset from each other in a plane parallel to the light emitting surfaces.