A backlight module is provided. The backlight module includes a frame, a light source and at least an optical film. The optical film has a first side with a first positioning flange and a second positioning flange, a second side with a third positioning flange and a third side with a fourth positioning flange. The edges of the first positioning flange, the second positioning flange, the third positioning flange and the fourth positioning flange abut the edges of the frame to hold the optical film in the frame.