A backlight unit of a display apparatus is provided. The backlight unit includes a display housing, a light source which is arranged along an inner circumference the display housing, a light guide plate through which light of the light source is transmitted, and which forms surface light, an optical sheet which is disposed on an upper side of the light guide plate to upwardly diffuse the light of the light source transmitted through the light guide plate, a plurality of first fixing units, each of which fix both the light guide plate and the optical sheet, a plurality of second fixing units which support only the light guide plate, and a support frame which is connected to the display housing to fix the optical sheet to the display housing provisionally, and has a window formed on a surface corresponding to the light guide plate to allow light to pass therethrough.