Epitaxial growth of aluminum nitride films by low pressure metal organic chemical vapor deposition is modeled. The feed gas mixture consists of trimethyl aluminum, ammonia, and hydrogen. The governing fluid flow, energy, and species conservation equations are solved numerically in two dimensions. The effects of process variables on growth characteristics is investigated in the mass transport limited regime. Results are presented in terms of flow patterns, species distributions, and growth parameters for various reactor pressures and growth temperatures. In the low pressure regime (50-150 Torr) the growth rate is nearly independent of temperatures between 600 and 1200 degrees C. The growth rate increases approximately linearly with pressure at 1200 degrees C.