A light guide plate (LGP) includes a first surface, a second surface, and a plurality of light incident surfaces. The first surface is a light-emitting surface. The light incident surfaces connect the first surface and the second surface. The second surface has a plurality of ring-shaped grooves. Depths of the ring-shaped grooves are gradually decreased from a place close to a center of the LGP towards places close to the light incident surfaces. Each of the ring-shaped grooves has a first ring-shaped side wall surface and a second ring-shaped side wall surface. The second ring-shaped side wall surface is opposite to the first ring-shaped side wall surface and located between a geometry center of the ring-shaped groove and the first ring-shaped side wall surface. The first ring-shaped side wall surface is a tilted surface. In addition, a light source apparatus is also provided.