Gas atornization experiments were carried out with Cu-20%Co alloy (mass fraction). Powders with a fine dispersion of the Co-rich particles were obtained. A model was developed to describe the solidification process of the atomized drops. The kinetic details of the microstructure evolution in the drops were calculated. It was indicated that the numerical results had a good accordance with the experimental ones. The microstructure development during the liquid-liquid decomposition was a result of the concurrent action of the nucleation, the growth of the minority phase droplets, the collision coagulations between the droplets and the spatial separation of phases. A bigger drop achieved a lower cooling rate and a lower nucleation rate of the minority phase droplets. It exhibited, therefore, a coarse Co-rich particle microstructure.