A backlight unit for a liquid crystal display device is provided to prevent a light guide plate or diffusion plate from deforming or displacing. The backlight unit comprises a light source; a plurality of optical sheets disposed between the light source and a liquid crystal display panel; a light guide plate disposed between the light source and the plurality of optical sheets; a cover bottom disposed below the light guide plate; and a guide holder fixed to edges of the cover bottom to hold a lower surface and sides of the light guide plate at peripheral of the light guide plate, wherein the guide holder includes a base block facing to the lower surface of the light guide plate and at least one side wall extended from the base block to be opposite to a side of the light guide plate.