An apparatus inspects internal defects of substrate, the substrate having an upper surface and a plurality of side surfaces connected with the upper surface. The apparatus includes at least one light source arranged on one of the side surfaces of the substrate and emitting a light beam on the corresponding side surface and into the substrate, the incident angle of the light beam is limited to a first predetermined angle within a range allowing the light beam to transmit in a total internal reflection manner in the substrate; an image capturing module arranged above the substrate to capture the image of the upper surface of the substrate, a light shield mask arranged between the image capturing module and the substrate and shielding an edge portion of the upper surface of the substrate.