c-Axis-oriented wurtzite (0 0 01) AlN layers are grown on a (111) diamond substrate by metal-organic vapor phase epitaxy. The microstructure and growth mechanism of the AlN layer are examined using Xray diffractometry and transmission electron microscopy. At the initial stage of AlN growth, AlN particles with a two-domain structure are nucleated on the (111) diamond surface. The orientation is either < 11 (2) over bar0 > AlN parallel to [1 (1) over bar0] diamond [named the AlNI domain] or < 10 (1) over bar0 > AlN parallel to [1 (1) over bar0] diamond with a rotation angle of 30 degrees from AlNI around the c-axis [named the AlNII domain]. The initial AlN particles are predominantly composed of AlNII. Subsequently, the AlNII grains incorporate the quite-low-density AlNI grains upon further growth. Finally, a continuous AlN layer dominated by the AlNII domain is obtained on the diamond. The growth dynamics is significantly different from that of the AlN layer grown on (111) Si. The growth mechanism is understood by considering the periodic atomic arrangement of AlN and (111) diamond. We conclude that AlNII on (111) diamond has a geometrically appropriate epitaxial relationship that reduces the stress energy of the AlN lattice. (C) 2009 Elsevier B.V. All rights reserved.