Abstract: The activity coefficients of vanadium in liquid copper were measured by equilibrating solid V₂O₃ with liquid Copper under different oxygen partial pressure in temperature range from 1673 to 1873 K, where the oxygen partial pressure was controlled by adopting the CO _(g)/CO_2(g) equilibrium. Because V₂O₃ exhibits non-stoichiometry of oxygen, the Gibbs energy model for (V₂O₃) _S.S., not that of pure V₂O₃ compound, was adopted to calculated the activity coefficients of vanadium in liquid copper. The activity coefficients of vanadium in liquid copper measured under the highest experimental oxygen partial pressure as a function of temperature can be expressed by a linear relationship. This relationship can also be considered as the temperature dependence of Henry's constant of vanadium in liquid copper.