Aluminum (Al) incorporation in Alx Ga1-x N films grown by low-pressure metal organic vapor phase epitaxy using trimethylaluminum (TMAl) and trimethylgallium as group III precursors has been systematically studied. The solid phase Al composition of the Alx Ga1-x N films varied nonlinearly with the Al gas phase composition. The incorporation kinetics of Alx Ga1-x N alloy has been analyzed by using an adsorption-trapping model. Two parameters were used to characterize the properties of Al incorporation, i.e., the capture radius and the adsorption time of Al atoms. An exponential function of the Al composition of the Alx Ga1-x N films versus the TMAl gas flow rate was obtained. It was demonstrated that the adsorption time of the Al atom was larger than the growth time of one atomic layer. The effects of ammonia flow rate, crystal growth rate, and growth temperature on the adsorption parameters were also discussed.