The spinel ferrite nanoparticles have potential in the industry and medicine, due to interesting and unusal properties. These properties depend on processing conditions, particle size, doping, chemical composition and A-B exchange interactions. The improvement of the magnetic properties of ferrites by doping with different ions is one of the purposes of scientistes. In this work, the sol-gel auto-combustion method was used to prepare nanocrystalline powders of Co-substituted nickel ferrite with the general formula NiCo x Fe 2-x O 4 ( x = 0.0, 0.1, 0.25, 0.5, and 0.75). This study consists of two parts. In the first part, we have investigated the effects of Co substitution on structural and magnetic properties of nickel ferrite and in the second part, the effects of sintering temperature on evolution of structural and magnetic properties of NiCo x Fe 2-x O 4 . The structural and magnetic properties of samples have been studied using analysis such as X-ray diffraction (XRD), Fourier transform infrared (FTIR), field emission scanning electron microscopy (FE-SEM), and vibrating sample magnetometer (VSM). The XRD patterns indicated that the crystalline structure is cubic spinel with Fd-3m space group. The MAUD results indicate that the synthesized nickel ferrite and NiCo x Fe 2-x O 4 ferrites have inverse and mixed spinel structure, respectively. FE-SEM images showed particles in the nanosized range. The VSM data showed that by increasing Co doping, the saturation magnetization (M s ) decreased slowly and the coercivity (H c ) increased significantly. The increasing of H c is attributed to the higher magneto-crystalline anisotropy of Co 3+ ions. The XRD patterns of the samples annealed at 500 ? C and 1000 ? C, showed the peaks width reduces by increasing sintering temperature indicating the crystallite size enhancement. The M s increases with increasing sintering temperature and can be caused due to two reasons: (1) High crystallinity and a low amount of microstructure defectes, (2) In the core-shell morphology, the higher core/shell ratio of large particles leads to eliminations of surface effects and consequently enhancement of M s . By increasing the sintering temperature, Co 3+ ions in the A sites, start distributing into the B sites and hence the number of Fe 3+ ions at B site is reduced. Consequently, the M s decreases with increasing sintering temperature. The competition between two factors above and redistribution cation led to decreasing M s . The H c decreases with increasing sintering temperature. The decrease of H c is proportional to 1/D. Increasing particle size lead to decrease of coercivity in multi-domain structure.