An optical film is applied to a side-emitting backlight module. The side-emitting backlight module has a light guide plate for guiding a chief light beam. The light guide plate has a light emitting surface to define a normal direction thereof. The optical film includes a lower prism structure, an intermedium layer, and an upper prism structure. The lower prism structure is disposed on the light emitting surface. The intermedium layer is connected between the lower prism structure and the upper prism structure. When the chief light beam exits from the light emitting surface of the light guide plate, and then goes through the lower prism structure, it goes through the intermedium layer along a changed traveling direction. When the chief light beam exits from the intermedium layer, and goes through the upper prism structure, it turns from the changed traveling direction to the normal direction.