This study theoretically investigates the influence of the growth orientation on In incorporation during metal-organic vapor phase epitaxy (MOVPE) growth. We propose a new theoretical model based on first-principles calculations that show the role of the N-H molecular layer on In incorporation. Under MOVPE growth conditions, III-nitride surfaces terminated by N-H molecular layers are stable. The N-H layer that covers the In atomic layer prevents In atom desorption and is replaced by Ga atoms. In incorporation is, therefore, more efficient for higher N-H layer coverage and stability. To investigate this relationship, calculation of the enthalpy change for the decomposition of an N-H molecular layer was performed. To take into account the experimental conditions, temperature dependence of surface reconstruction is considered. The trend of this enthalpy change depends on the growth orientation, which agrees well with the experimental In composition.