A growth model for atomic layer epitaxy (ALE) of GaAs by pulsed laser metal organic vapor phase epitaxy is reported. The physical processes of the model are surface reactions: the adsorption, desorption and decomposition of trimethylgallium (TMG) or triethylgallium (TEG) . These surface processes are expressed in terms of time constants Different decomposition time constants are assumed for TMG/TEG adsorbed on Ga and As terminated surfaces. It is found that an enhancement of the decomposition rate of TMG/TEG adsorbed on As compared to that on Ga terminated surfaces caused by laser irradiation is the main origin for the suspension of Ga deposition at 100% coverage.