This paper has studied the electrical and interfacial properties of atomic-layerdeposited Al 2 O 3 thin film on ammonium-sulfide passivated GaAs. It was found that the Al 2 O 3 deposited at 300°C relative to that at 100°C showed the nearly four orders of magnitude reduction in gate leakage current at the capacitance-equivalent- thickness of 40 Å. The capacitance-voltage (C-V) characteristics displayed the higher oxide capacitance, reduced frequency dispersion and less charge trapping when GaAs receiving (NH 4 ) 2 S sulfide immersion; these improvements can be reasonably explained by the suppression of both native oxides and the resultant improved interface quality. Annealing as-deposited Al 2 O 3 /GaAs structures at high temperatures further reduces the Fermi level pinning effect on accumulation capacitance, however, causes an increase in C-V frequency dispersion and gate leakage current. We suggested that these phenomena are strongly associated to the amount of As-related defects resided at the dielectric/substrate interface during thermal desorption.