Spinel ferrites are a group of materials in the nanometer scale which have a wide variety of applications in medicine and economics. Now, of the problems in this area is fabricating pure homogenous nanoparticles with controllable size. In this research, in order to prepare a pure sample of Ni ferrite in low temperatures, several methods including sol-gel with changing parameters such as pH, complexion agent and co-precipitation methods were used. In all the synthesized samples, the effect of temperature on the structure and magnetic properties of the samples, was studied using analysis such as x-ray diffraction (XRD), Furier Transform Infrared (FTIR), microscopic imaging (SEM) and vibrating sample magnetometer (VSM). The x-ray diffraction spectrum of the samples prepared via the sol-gel method and with different pH, verified that increasing pH, decreases the formation temperature of the phase of pure ferrite nickel in this method. Also, the results showed that increasing pH of the first solution to 7 is concomitant with decreasing the nano-particles size and in the alkaline solution (pH 7), again the size of the particles increases. Also, in samples synthesized by the co-precipitation method, it was observed that temperature increase, causes the formation of the second phase of ?-Fe ?? O ?? in the samples. Finally, utilizing the complexing agent of EDTA in synthesizing the samples by the sol-gel method, we managed to fabricate pure samples of nickel ferrite in low temperatures. The complexing agent EDTA, by surrounding the metallic ions, not only decreases the probability of oxidation of metallic ions, but also decreases the interaction between ions and causes the formation of finer particles. The effect of temperature on the samples was studied. The results of XRD showed that increasing the annealing temperature, results in larger particles. The SEM images also confirm the enlarging of particles concomitant with temperature increasing. Also, strain in these samples decreases along with annealing temperature decreasing, which is a sign of structure conversion from inverse spinel to mixed spinel. The inverse structures of the samples, is changed and with the redistribution in the arrangement of the cations in the tetrahedral and octahedral sites, the mixed structure is formed. The changes in the relative intensity of the Brag peaks confirms this idea. The magnetization of the samples, demonstrate the increase in saturation magnetization with enlargement of the particles. Based on the core-shell model, every particle is formed of a core part with the magnetic order, called core, and a disordered shell layer, named shell. Along with the particles enlargement, the contribution of the core increases relative to the shell, and therefore, the saturation magnetization of the samples increases too. Albeit, the structure conversion of the inverse to the mixd spinel, has affected the magnetization. It was also observed that the coercive field of the samples increased with increasing the size of the particles up to 33 nm, and then, with more increasing in the size, the field decreased. The maximum of the coercive field, happens in the critical size of the single-domain, and this critical size for the samples prepared by the EDTA method happened in the range of 25-33 nm. Keywords: Spinel ferrites, Sol-gel, Precipitation, pH, Temperature effect, EDTA, Cation Distribution, Core-shell model, Critical size.