A light guide plate includes a body region and a light incident region. The body region has a light exiting surface, an opposite base surface, and a side surface between the light exiting surface and the base surface. The light incident region is located at the side surface of the body region for receiving incident light and includes projections protruding outwards from the side surface of the body region, and a cavity is defined between adjacent projections. Each projection has a top surface and outer side walls. The outer side walls also define inner side walls of each cavity together with a bottom surface. The top surface, side walls, and bottom surface together are a light incident surface of the light incident region. An angle between the top surface and each outer side wall is called an apex angle, which is greater than 90 degrees.