A light guide plate including a body and a plurality of prism microstructures is provided. The body has a bottom surface, a light emitting surface opposite to the bottom surface, and a plurality of side surfaces. The light emitting surface has a central area and at least one peripheral area outside the central area. The prism microstructures are disposed on the light emitting surface and the bottom surface. The prism microstructure disposed on the light emitting surface includes a plurality of first prism microstructures in the central area and a plurality of second prism microstructures in the peripheral area. A top angle θ2 of the second prism microstructure is greater than a top angle θ1 of the first prism microstructure. A backlight module including the above-mentioned light guide plate is also provided.