Abstract: Cobalt ferrite Co_xFe_3-xO₄（x=0.2~1.0） nanocrystalline powders were prepared by chemical coprecipitation method and annealed at temperatures from 1260℃ to 1340℃ in air. The structural and magnetic properties of the samples were analyzed through the X-ray diffraction（XRD）patterns and vibrating sample magnetometer（VSM）. The experimental results show that monophase Co_xFe_3-xO₄ ferrites with a spinel structure are formed with high Co²⁺ content（x≥0.7）, while with the lower Co²⁺ content（x＜0.7）Co_xFe_3-xO₄ phase and α-Fe₂O₃ phase are obtained at the same time. The specific saturation magnetization climbs up at first and falls down with the cobalt content increasing. The maximum of σ_s is attained at the x=0.8. In the range of x=0.5~0.8, the coercivity H_c decreases with the increase of x, then climbs up rapidly. Higher σ_s and H_c can be achieved simultaneously at about x=1.0.