In recycling a lot of low grade ferrous scraps, we cannot remove tramp elements, such as copper and tin, which are inevitably dissolved in molten iron. Accordingly, the thermodynamic data between copper and alloying elements in molten iron are necessary to know the influence of copper on the property of steel. However, the available data are not enough because of the difficulty of the measurements. One of the purposes of this study is to develop a method to measure the thermodynamic data of tramp elements in iron. In this work, we have tried to measure the oxidation equilibria of copper in molten iron by using silver as an intermediate phase. By applying this method, the interaction between Cu and M(M: B, Co or Ni) in molten iron has been investigated at 1873 K.