Nickel-substituted cobalt-zinc nanoferrites (Co_0.5−xZn_0.5Ni_xFe₂O₄, x = 0.05 to 0.25 in steps of 0.05) were synthesized in a citric acid auto-combustion method. Divalent strong paramagnetic nickel ions substituted in place of ferromagnetic cobalt ions resulted in slight modifications over structural and significant changes in magnetic properties of the system. All samples were characterized with X-ray diffraction technique for structure determination. Crystallite sizes were calculated using the Williamson-Hall plot and size-strain plots, from these lattice strain and microstrain were discussed. Transmission electron microscopy measurements were done for the base sample (x = 0), and they showed well-developed spherical particles with an average size of 35 nm. SEM micrograph was recorded for the base sample to observe surface morphology, and it was observed to be stoichiometric, as evident from EDAX measurements. Saturation magnetization, remanence, coercivity and remanence ratio were measured using M-H loops recorded using a vibrating sample magnetometer at room temperature. Magnetocrystalline anisotropy was discussed with the help of magnetocrystalline anisotropy constant (K₁) obtained from the law of approach to saturation method.