The entrapment of gas bubbles and non-metallic inclusions such as alumina by the solidifying shell in the continuous casting of steel is known to be determined by the interfacial tension gradients at the solid-liquid interface. Mukai and Lin1-3) suggested a mechanism of the entrapment of particles at the solid-liquid interface, and Zeze and Mukai4) confirmed that this model acceptable for the prediction of the behaviors of bubbles and inclusions in stainless steels. In this model, oxygen and sulfur are the most important elements determining the entrapment behavior. In the present study, the effect of oxygen on the surface tension of liquid Fe was investigated with the constrained drop method. By combining the present results with Young's equation, the interfacial tension was obtained as a function of the oxygen content in liquid Fe. It was considered that a gas bubble at the solid-liquid interface is entrapped, whereas an alumina inclusion is pushed away.