Stress evolution during metal organic chemical vapor deposition growth of AlN layers on (111) Si and (0001) sapphire substrates was investigated using in situ wafer curvature measurements in order to understand the origin of growth stresses. AlN layers 170±30nm thick were deposited over a temperature range of 600–1100°C at a growth rate of 0.2±0.05nm/s. On (111) Si, AlN films grow under a constant tensile stress right from the beginning of growth in the temperature range investigated. In contrast, above 900°C on sapphire, an initial compressive growth stress is observed followed by a transition to a final tensile stress, while below 800°C the stress is tensile right from the beginning of growth as observed on Si. The origin of this stress behavior is explained in terms of a combination of epitaxial stress and grain coalescence stress. Calculation of the grain coalescence stresses show that the value is higher for growth on sapphire substrates than on Si substrates. Also, for growth on both substrates, a sharp drop in the value of the coalescence related tensile stress is observed between 900°C and 800°C. Both these features are correlated to the variation in the integral breadth of the (0002) and (1011)AlN x-ray rocking curves. It is shown that both the lower tensile stress on Si substrates and the drop in stress between 800°C and 900°C can be attributed to the reduction in grain coalescence stresses due to increasing grain boundary energies in the film resulting from increasing grain misorientation with respect to the substrate/film interface.