The thermodynamic behavior of copper in molten Fe-C-S alloy at high temperature was investigated by chemical equilibrating technique. The effects of copper, carbon, sulfur content and temperature on the activity coefficient of copper in molten pure iron and carbon saturated were was determined by measuring the distribution ratio between silver and Fe-C-S-Cu alloy. The activity coefficient of copper in molteniren and carbon saturated iron at infinite dilution of copper were measured 11.02 and 28.50 at 1823K respectively. The interaction parameter between copper and copper, sulfur and carbon in molten iron at 1823K, was estimated as follows; ε^(Cu)_(Cu) = -4.80, ε^S_(Cu) = -2.544, ε^C_(Cu) = -4.60 The dependence of the activity coefficient in molten Fe-C alloy on temperature was determined as follows; For molten pure iron : lnγ^(oFe)_(Cu) = 4370/T For Fe-2wt%C : lnγ^(oFe-2wt%C)_(Cu) = 0.72 + 3840/T For carbon saturated iron : lnγ^(oFe_C)_(Cu) = 1.11 + 4100/T Combining the effect of temperature and the alloying component, the activity coefficient of copper could be expressed as follows ; lnγ^(Fe-C-S)_(Cu) = 4370/T·(1-X_(Cu))² + (2.15 + 4600/T)·2.54·X_s The possibility of removal of copper from steel by slag refing and evaporating technique was discussed.