The cracking mechanism in AlN(11 (2) over bar0)/alpha -Al2O3(1 (1) over bar 02) heteroepitaxial film grown by MOCVD is discussed. The crystal structure and microstructure of an AlN/Al2O3 film and an AlN/GaN/Al2O3 film are compared using high-resolution X-ray diffractometry, optical microscopy, scanning electron microscopy, and transmission electron microscopy. In the AlN/Al2O3 film, cracks parallel to the [1 (1) over bar 00](AlN) direction and perpendicular to the interface of the film and the substrate are observed. The cracks do not propagate to the AlN film surface. The tips of the cracks are widest in the AlN film, and the cracks narrow as they penetrate deeply into the substrate. On the other hand, in the AlN/GaN/Al2O3 film, no cracks are observed. A concave curvature is observed in the AlN film with cracks on the Al2O3 substrate along the [0001](AlN) direction, whereas a convex curvature is observed in the AlN film without cracks. On the basis of these results, the cracks, formed in the AlN film due to the tensile stress along the [0001](AlN) direction during the epitaxial growth, propagate to the AlN film surface and into the Al2O3 substrate. On the other hand, in the AlN/GaN/Al2O3 film, it seems that the GaN buffer layer suppresses the tensile stress; as a consequence, no cracks occur.