The crystal structures and microstructures of AlN(11 (2) over bar0)/GaN(11 (2) over bar0) epitaxial films on just-cut and +/-4degrees off-cut Al2O3(1 (1) over bar 02) substrates grown by metal organic chemical vapor deposition (MOCVD) are investigated using high-resolution X-ray diffractometry and transmission electron microscopy, and are compared with those of AlN(11 (2) over bar0) film on +4degrees off-cut Al2O3(1 (1) over bar 02) substrate. In the AlN/Al2O3(+4degrees off-cut) film and the AlN/GaN/Al2O3 (just-cut, -4degrees off-cut) films, cracks parallel to the [1 (1) over bar 00](AlN) direction and perpendicular to the interfaces of the films and the substrates are observed. The AlN/Al2O3 and AlN/GaN interfaces exhibit low crystallinity in which moire fringes are observed. On the other hand, in the AlN/GaN/Al2O3(+4degrees off-cut) film, no cracks form. The GaN layer buffers the lattice mismatch between the AlN film and the Al2O3 substrate, and moire fringes are not observed in the GaN/Al2O3 and AlN/GaN interfaces. On the basis of these results, the effects of the interface structures on cracking are discussed.