A method of forming a backlight assembly is presented. The invention reduces the degradation of display quality caused by an uneven expansion of the reflective sheet in a display device. the reflective sheet may be positioned between the light guiding plate and the base in a backlight assembly of a display device. The backlight assembly includes a light source, a light guiding plate positioned to guide light from the light source in a predetermined direction, a base on which the light guiding plate is mounted, and a reflective sheet positioned between the light guiding plate and the base. The reflective sheet has a cutout. A supporting structure extends through the cutout between the light guiding plate and the base to support the light guiding plate. A liquid crystal display device made with such backlight assembly is also presented.