In order to prepare spherical particles of cobalt powders and add Y2O3 into cobalt powders, the preparation thermodynamic of the precursor of Co-Y2O3 composite powders by coprecipitation was analyzed. The lg[M]T-pH diagram was obtained by analyzing the thermodynamics of Co2 -Y3 -NH3-C2O42--H2O precipitation system. It is shown that the complexation of Co2 ions is promoted by increasing the total concentration of oxalic acid root [C]T and ammonium root [N]T could. The precipitation rate of Y3 ion is only influenced by the total concentration of oxalic acid root [C]T and declines with its increase. The optimum pH values for the precipitation of Co2 and Y3 are 2~2.5 and 1.5~2.5, respectively. The precursor of Co-Y2O3 composite powders was prepared by coprecipitation with the pH value of 2.5. The shape of the precursor composite powders is cluster ball with good dispersion. The spherical Co-Y2O3 composite powders with uniform size distribution and the mean particle size of about 5 μm were obtained.