A backlight module includes a back plate, an optical sheet, and a lateral frame. The back plate has a first surface, wherein the first surface includes a raised portion formed near the edge of the first surface. An insertion opening is formed between the raised portion and the first surface. The optical sheet is disposed on the first surface, wherein a protrusion is formed at one end of the optical sheet to be inserted into the insertion opening. The lateral frame is disposed on the back plate and corresponds to the edge of the first surface. The lateral frame includes at least a side wall having a sinking portion formed at the bottom of the side wall, wherein the bottom of the side wall is disposed on the optical sheet while the raised portion is received in the sinking portion.