The solvus phase boundaries of the Co-Cu system were recalculated by taking into account the effect of copper on the magnetic free energy of alpha -Co. It was confirmed that the solubility of copper in alpha -Co is significantly lowered owing to the magnetic ordering energy below the Curie temperature Tc and there is an anomaly in the Arrhenius plot of solubility. The precision lattice parameter determination by X-ray diffractometry and the intrinsic coercivity Hci measurement by SQUID was performed and the calculated results were justified. Two different thermodynamic treatments of magnetic free energy were also compared in this study.