Experiments were carried out at 1　873 K in a double-temperature zone furnace using vapor pressure method. The equilibrium tests of barium vapor with liquid iron were conducted in CaO crucibles sealed in a Mo chamber. As a result, the experiments obtained the relation between the dissolved content of Ba in liquid iron and the vapor pressure of Ba, and the standard Gibbs energies for the reactions Ba(g)=[Ba] and Ba(l)=[Ba].