An optical film includes a substrate and a plurality of grating structures. The substrate has a light incident surface and a light exit surface opposite to the light incident surface, wherein the light exit surface includes a plurality of semi-cylinder surfaces disposed side by side. The grating structures are disposed on portions of the light exit surface, and an extending direction of each of the grating structures is substantially parallel with an extending direction of each of the semi-cylinder surfaces. The optical film improves uniformity of a plane light source. Further, a backlight module using the optical film is provided.