In this thesis, cetyltrimethylammonium bromide (CTAB) was used as surfactant. C4H6CoO4·6H2O and FeCl3·6H2O were used as raw materials. The crystallization temperature was 180 ℃, the crystallization time would last 8 h and the solution was passed at pH=11. Cobalt ferrite nanoparticles were prepared by hydrothermal method. The doped rare earth elements Gd3+ was used to control the structure and absorbing properties of cobalt ferrite, and at the same time explore the optimal doping amount of rare earth elements. The structure and absorbing properties of cobalt ferrite were characterized by X-ray diffraction (XRD), fourier transform infrared (FTIR), transmission electron microscopy (TEM) and vector network analyzer (VNA). It was found that during cobalt ferrite preparation, the doping of rare earth elements Gd3+ would have a great influence on grain size, morphology and absorbing properties. The results showed that when the doping amount of rare earth elements was x ≤ 0.1, the decrease of the grain size of cobalt ferrite and increases of the lattice constant gradually with the increase of doping amount, indicating that rare earth elements can make the particles smaller. The absorbing reflectivity of cobalt ferrite reached the minimum values of -14.9 dB when the doping amount of rare earth elements Gd3+ was 0.025.