Amorphous aluminum oxide films were prepared by rf-sputtering at various values of the pressure of sputtering atmosphere, and their density, refractive index, Young＇s modulus and internal stress were measured. The physical properties of the present films depended on the pressure of sputtering atmosphere. The density, refractive index, and Young＇s modulus decreased with the pressure below about 6.5 Pa, beyond which they increased. The compressive stress and tensile stress were induced in the films depending upon the pressure of sputtering atmosphere, and the tensile stress reached a maximum at the pressure of 6.5 Pa and then decreased as the pressure increased. From the results of the energy-dispersive X-ray analysis (EDX), it was found that the pressure of sputtering atmosphere gave a large influence to the chemical composition (the atomic ratio, O/Al) of the film. The pressure dependence of the physical properties was successful to be explained by the change of chemical compositions of the films. The presence of OH group in the films was verified by using FT-IR, and their microstructure was investigated by SEM study.