To investigate and distinguish the difference of Cs activation mechanism on Al0.5Ga0.5As (001) and Al0.25Ga0.75N (0001) surface, plane wave with an ultrasoft pseudopotential method based on the first-principle density functional theory is used. Surface adsorption energy, work function, and surface dipole moments with different Cs coverage are calculated and compared. Eight possible Cs adsorption sites are chosen for the Al0.5Ga0.5As (001) surface, while seven high-symmetry sites are considered i