A light guide plate (LGP) and a light source module are provided. The LGP includes a plate body, first microstructures and second microstructures. The plate body has a first light incident surface, a second light incident surface adjacent to the first light incident surface and a first surface connected thereto. The first microstructures are located on the first surface, and each has a first optical surface facing the first light incident surface. Two sides of each first microstructure respectively have a first edge portion. One of the first edge portions faces and is inclined relative to the second light incident surface, and there is a first included angle between the first edge portion facing the second light incident surface and the first surface. The second microstructures are located on the first surface, and each second microstructure has a second optical surface facing the second light incident surface.