A surface light source assembly includes at least one light emitting element, a light guide plate and a reflective sheet. Each of the light emitting elements has an annular light emitting side surface. The light guide plate has a bottom surface and a light exit surface opposite to the bottom surface. The bottom surface has at least one accommodating recess to accommodate at least one light emitting element. Each of the accommodating recesses has a light incident surface. The bottom surface further has a flat portion and at least one inclined surface portion. Each of the inclined surface portions is connected between the flat portion and the light incident surface of the corresponding accommodating recess. The reflective sheet is disposed below the flat portion. A light source module having the surface light source assembly is further provided.