Cobalt bearing magnetite particles were synthesized by aerial oxidation of ferrous and cobalt ions at alkaline condition and room temperature. The samples obtained at different time intervals during reaction were subjected to XRD and IR, and it was indicated that the cobalt-bearing ferrites were produced through green-rust intermediate phase. Different compounds were synthesized using 1/20, 1/15, 1/10, 1/7, 1/5 and 1/3 Co2+/Fe2+ starting ratios, and Co-ferrites with Co/Fe of 0.05, 0.06, 0.08, 0.12, 0.17, 0.18 were obtained. The chemical composition, microstructure and micrograph of those precipitates were characterized by chemical analysis, IR, XRD and SEM. The results revealed that Co2+ replaced Fe3+ at octahedral position in spinel structure, and the increase of Co in initial solution showed deleterious effect on the crystallinity of the precipitates.