A liquid crystal display device includes a liquid crystal panel, at least one optical sheet substantially rectangular in shape, where first and second openings are formed on the end edges of one side of the optical sheet at a predetermined interval, a main frame in which the optical sheet(s) are placed, and a sub-frame arranged above the main frame, in which the main frame is provided with first small protrusions jutting upward corresponding to the first openings at the top surface, while the rear surface of the sub-frame is provided with second small protrusions jutting downward corresponding to the second openings, thereupon the first small protrusions are inserted into the first openings of the optical sheet(s) and the second small protrusions are inserted into the second openings to connect and install the sub-frame to the main frame, and the optical sheet(s) are then aligned and fixed. Accordingly, the optical sheets are aligned better and firmly fixed to achieve larger display screens and thinner liquid crystal display devices.