A backlight module includes a light guide plate having a bottom surface, a light emitting surface, a first light incident surface, and microstructures. Each microstructure recesses into or protrudes from the bottom surface and has a first and a second surfaces. The first and the second surfaces of at least one of the microstructures are located on two sides of a first reference plane parallel to the first light incident surface. A section-line of each first surface on a second reference plane perpendicular to the first light incident surface and perpendicular to the light emitting surface is a straight line. A first angle between each first surface and a third reference plane parallel to the light emitting surface in the light guide plate is between 0 degrees and 20 degrees, and a thickness of each microstructure is between 0 micrometers and 20 micrometers.