The surface states of Al0.7Ga0.3 As(l00) were studied by ultraviolet photoelecton spectro-scopy. It was found that there exist two surface states on Al0.7Ga0.3 As (100) surface, which could be removed by the adsorption of 1500 L hydrogen. The evolution of these two surface states with thermal annealing was investigated. Combining with the experimental results of LEED and XPS, it was recognized that the damage of surface could be removed effectively by a thermal annealing at a temperature near 450℃, and a nearly perfect surface of A10.7Ga0.3 As(l00) could be obtained.