A backlight unit of a liquid crystal display device and a method of assembling the same which makes it easy to inspect for impurities and for reducing damage to an optical sheet. The backlight unit includes a lamp assembly; a light guide panel for providing the light from the lamp assembly incident to a display panel; and a bottom cover for supporting the lamp assembly and the light guide panel, wherein the distance between the lamp assembly located at opposite edges of the bottom cover is longer than the length of the light guide panel.