In the present study, the surface tensions of pure liquid Al, Si, and liquid Al-Si alloys were systematically investigated by means of the oscillating drop method in combination with electromagnetic levitation. Prior to the measurement, the ambient oxygen partial pressure in the chamber was measured using an yttrium-stabilized zirconia oxygen sensor to estimate the surface oxygen partial pressure based on the thermodynamic assesssment. Under the experimental Ar gas with oxygen partial pressure of 10(-1) Pa, the oxygen-reduced surfaces of pure liquid Al and Si can be obtained. However, the surface tensions of alloys in Al-rich composition are strongly affected by the oxygen adsorption under the same oxygen partial pressure. With the increasing Si concentration, the surface tension of the alloy approaches to the values of the oxygen-reduced surface.