By using the density functional theory of quantum chemistry, the gas reaction mechanism in the AIN MOVPE process has been investigated, especially after the amide DMAlNH2 formation. Two reaction paths are distinguished after the amide DMAlNH2 formation and oligomerization: the intramolecular path and the intermolecular path, both involved with methane elimination. By inspections of the changes of the Gibbs energy AG between products and reactants, as well as the Gibbs energy of activation divided by RT, Delta G*/RT, to account for thermal activation at different temperatures, the most probable gas reaction paths, and gas products for AlN thin film growth are determined both thermodynamically and kinetically. Our results indicate that under metal organic vapor phase epitaxy condition, for the intramolecular path, (MMAlNH)(2) is the most probable gas reaction products; for the intermolecular path, both Al(NH2)(3) and (AlNHNH2)(2) are the most probable gas reaction products. We also prove that (AlN)(2) and (AlN)(3) clusters are thermodynamically unfavored in the gas phase.