An aluminum nitride single-crystal multi-layered substrate comprising an aluminum nitride single-crystal layer formed by direct reduction nitridation on a single-crystal .alpha.-alumina substrate such as a sapphire substrate and an edge-type dislocation layer having a thickness of 10 nm or less in the vicinity of the interface between the both crystals. Threading dislocation is rarely existent in the aluminum nitride single-crystal layer existent on the surface. It is useful as a semiconductor device substrate.