Abstract: The isoactivity of [Y] of samples was obtained in multiple- hole graphite crucibles by controlling the oxygen potential through the reaction 2 [Y]+3CO = Y2O3+3C. The activity interaction coefficients between Y and Sn. Zn, Pb alloying elements were determined by using coniugate gradient method of solving ill-conditioned equations. The equilibrium constant of the reaction 2 [Y]+3 [O] = Y2O3 and the standard free energy of formation of Y2O3 in liquid Cu were obtained at 1200℃