A light guide plate includes a light guide layer and a first microstructure layer. The light guide layer has a light exit surface, a back surface, and a light incidence surface interconnecting the light exit surface and the back surface. The first microstructure layer is disposed on the back surface, and includes a plurality of first microstructures spaced apart from one another. Each first microstructure has a polygonal planar base, and two first inclined faces and two second inclined faces which extend from the planar base, and which are proximal and distal to the light incidence surface, respectively. Each first inclined face intersects the respective second inclined face at a respective first ridge. Each first ridge cooperates with the planar base to form a first angle that ranges from 5 to 70 degrees.