The structure and magnetic properties of Co 1−X Cu X Fe 2 O were investigated. Cobalt copper ferrites have been synthesized by the double sintering ceramic technique. X-ray diffraction analysis and transmission electron microscopy (TEM) confirmed the formation of single-phase cobalt ferrite nanoparticles. The particle size was estimated by TEM micrography to be 46, 51, 40 and 36 nm for X = 0.0, 0.1, 0.3 and 0.6, respectively, which is a good result for this method of preparation. The magnetic susceptibility was studied at different temperatures as a function of magnetic field intensities. The room temperature hysteresis loop was found for the present samples in a field intensity of 7 kOe using a vibrating sample magnetometer. The parameters of x-ray density (d x 4 ) and apparent density (d ) increase with increasing Cu 2+ concentration in the prepared samples. a