The invention provides a light source module including a light guide plate and light emitting elements. The light guide plate includes a main plate body and a plurality of optical microstructures. The main plate body has a light emitting surface and a back surface opposite to each other, and a light incident surface connected therebetween. The optical microstructures are formed on the back surface. Each optical microstructure includes at least two sections connected to each other, each section having a reflective surface. The light emitting elements are disposed on the light incident surface, and light emitted by each of the light emitting elements is reflected by at least some of the reflective surfaces and transmitted to the light emitting surface. In any optical microstructure, the reflective surfaces are not parallel to each other.