Abstract: The activity coefficients of vanadium in liquid iron were measured using chemical equilibrium method. This method consists of two slag-metal equilibria conducted under the same conditions. One is slag-metal equilibrium between vanadium-containing slag and liquid copper, and the other is equilibrium between liquid iron and the same compositional slag. The same compositional slags, atmosphere and temperature were adopted, so that the activities of vanadium oxide in the two slags are identical. Consequently, the activity of vanadium in liquid iron and copper that equilibrated with the slags are equal to each other. If the activity coefficient of vanadium in liquid copper is determined, the activity coefficient of vanadium in liquid iron can be known. By adopting the method mentioned above, the activity coefficients of vanadium were determined. When the concentration of vanadium in liquid iron are 0.28, 0.35 and 0.43 mass pct., the measured activity coefficients are 0.07, 0.08 and 0.09, respectively.