Wurtzite AlN layers are grown on a (0 0 1) diamond substrate by metal-organic vapor phase epitaxy. The microstructure and growth mechanism of the AlN layer are examined using atomic force microscopy, X-ray diffractometry, and transmission electron microscopy. At the initial stage of AlN growth, AlN crystalline particles with various orientations are randomly nucleated on the (0 0 1) diamond surface. At the second step of growth, AlN grains predominantly oriented along the c-axis grow, incorporate the randomly oriented AlN grains, and then grow further. At the final step of growth, a continuous AlN layer with a c-axis-oriented two-domain structure is obtained on the (0 0 1) diamond substrate. Microstructural analysis reveals that either the < 1 1 (2) over bar 0 > AlN domain or < 1 0 (1) over bar0 > AlN domain is aligned on the [1 1 0] diamond direction. The growth mechanism governs by the higher growth rate of the AlN grains along the [0 0 0 1] direction than along other directions at high growth temperatures up to 1270 degrees C. (C) 2009 Elsevier B.V. All rights reserved.