A thermodynamic model was applied to predict the GaBN MOVPE growth such as the equilibrium partial pressures, vapor-solid distribution relation, and growth efficiency. The delta-lattice-parameter (DLP) method was employed to determine the activities of the binary compounds. The large structural dissimilarity between BN and GaN results in a high interaction parameter of 70730cal/mol, indicating that GaBN deviates greatly from an ideal solid solution. A V/III ratio > 100 is required for the growth of GaBN at 1000 degrees C to avoid Ga droplets on the growth front. The growth of GaBN is controlled by the much higher Ga partial pressure than that of B over the alloy. The unstable regions in which inhomogeneous compositions are formed exist in the GaBN alloy based on the vapor-solid distribution relation. The predicted growth behavior of GaBN agrees well with the experimental data.