The spherical particles composed of cobalt carbonate and hydrated ferric oxide were prepared by the interfacial reaction method, in which the cobalt carbonate and the ferric oxide were formed by the reaction of mixture of aqueous solutions of cobalt sulfate and ferrous sulfate emulsified in benzene with sodium bicarbonate aqueous solution. The X-ray diffraction, the water vapor adsorption, the differential thermal analysis, and the particle size distribution were measured for the cobalt carbonate-hydrated ferric oxide powder. The crystal structure of the heat-treated powder and the reduced powder also examined by the X-ray diffraction method. From the results, it was concluded that the spherical particles composed of cobalt carbonate and hydrated ferric oxide were composites of spherocobaltite micro crystal and amorphous hydrated ferric oxide, and were highly porous similar to the spherical particles of cobalt carbonate or hydrated ferric oxide, both of which were prepared by the interfacial reaction method. It was also found that the spherical particles of cobalt ferrite or cobalt-iron alloy were formed by heat-treating or reducing the hydrated ferric oxide-cobalt carbonate spherical particles at high temperature, respectively.