A backlight module can be used in a dual display module and capable of blurring the shadow image produced by the sub display window in the main display window, while the main display window is displaying images, is disclosed. The disclosed backlight module comprises: a light emitting unit, a first frame located on one side of the light emitting unit and a first display window is enclosed by the first frame, a second frame located on the other side of the light emitting unit opposing to the first frame and a second display window is enclosed by the second frame, and a light diffusing plate, wherein the display area of the second display window is larger than the display area of the first display window. Besides, the light diffusing plate has a smooth surface and a rough surface respectively, while the rough surface is facing the light emitting unit.