Expansion of the system for thermodynamic parameters in liquid iron is expected for the improvement in iron & steelmaking processes. The development of their calculation method is one of the issues for its realization. In the present work, we proposed a calculation method for an activity coefficient of solute in infinite dilute liquid iron to element i, γ^ο_i, based on the surface tension of binary liquid Fe alloys. It was found that the estimated values by our proposed method agree with recommended literature data.