Growth behavior of AlGaAs heteroepitaxial layers on GaAs (111)Aplanes has been studied for atmospheric pressure organometallic vapor phase epitaxy. The Al composition as well as the arsine partial pressure and the growth temperature are the influential factors for the surface morphology of epitaxial layers on (111)Aplanes. By increasing the Al composition of grown layers, the range of growth conditions for obtaining a smooth surface morphology does not change so much. But the range for controlling defect structures, such as pyramidal shape, shifts to lower arsine partial pressures, and/or higher growth temperatures. The Al0.3Ga0.7As growth rate remains constant even at a lower arsine partial pressure than for the GaAs growth. This indicates that the morphology change is due to the increase in the arsine dissociation efficiency when the trimethylaluminum partial pressure increases.