Disclosed are a backlight unit, an assembly structure of which is modified to minimize the thickness of a bezel and to prevent optical defects, such as light leakage, and a liquid crystal display device using the backlight unit. The liquid crystal display device includes a liquid crystal panel, at least one light source fixing unit on which plural light sources are mounted and which is fixed to the inner side surface of a bottom cover, a diffusion plate receiving light from the plural light sources through a light entry plane and changing the progress direction of light into a forward direction, a sheet guide formed on the front surface of the bottom cover in a shape in which the inside of the sheet guide is opened, and plural optical sheets disposed on the sheet guide and separated from the diffusion plate to perpendicularly emit light from the diffusion plate.