Nowadays, with the development of industry and technology, spinel ferrite nanoparticles have become widespread because of their unique characteristics in medicine and industry fields. In this thesis, the structural and magnetic properties of CoFe 2-x Co x O 4 nanoparticles have been investigated. The hydrothermal method was used to synthesis samples. The hydrothermal method is one of the most powerful and popular down to up methods for nanostructure production. This research consists of three parts. The first part involves the production of cobalt ferrite nanoparticles and studying the effect of cobalt substitution on the structural and magnetic properties of CoFe 2-x Co x O 4 ferrite nanoparticles with (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6). After preparing the samples, X-ray diffraction (XRD), Ultraviolet-Visible Spectroscopy (UV-Vis), Field Emission Scanning Electron Microscopy (FESEM), and Vibrating Sample Magnetometer (VSM) measurements were used to characterize structural and magnetic properties of the samples. The XRD showed a spinel structure with the Fd-3m space group, and an impurity phase was also observed for x ? 0.5 samples. The XRD data analysis using the MAUD software confirmed the lack of a clear order in the behavior of the lattice parameter. This disorder is attributed to the distribution of cations in the cobalt-iron spinel ferrite network. The FESEM micrographs show that the samples are in nanometer scale sizes. The results of VSM analysis for x 0.4 samples have an irregular procedure both in saturation magnetization values and also coercive field. However, the impurity phase is vanished benefiting heat treatment of samples in 1000 °C, and the magnetization of the samples illustrates an ordered decreasing procedure. The decrease in the magnetization is due to the distribution of cations and also the less magnetic momentum of the Co 3+ compared to the Fe 3+ in B sub-lattice.