A backlight unit for a liquid crystal display device includes a bottom cover having a first plane portion and a first side portion combined by a first coupling system, a reflector over the bottom cover and having a second plane portion and a second side portion corresponding to the first plane portion and the first side portion, respectively, at least one cold cathode fluorescent lamp over the second plane portion, an optical sheet over the at least one cold cathode fluorescent lamp, and a guide panel overlapping a rim portion of the optical sheet, wherein the guide panel and the bottom cover are combined using a second coupling system.