Nano-sized nickel substituted cobalt ferrite powders, Ni_xCo_1-xFe₂O₄ (0.0 ≤ x ≤ 1.0), were fabricated by the sol-gel method, and their crystallographic and magnetic properties were studied. All the ferrite powders showed a single spinel structure, and behaved ferrimagnetically. When the nickel substitution was increased, the lattice constants and the sizes of particles of the ferrite powders decreased. The Mössbauer absorption spectra of Ni_xCo_1-xFe₂O₄ ferrite powders could be fitted with two six-line subspectra, which were assigned to a tetrahedral A-site and octahedral B-sites of a typical spinel crystal structure. The increase in values of the magnetic hyperfine fields indicated that the superexchange interaction was stronger, with the increased nickel concentration in Ni_xCo_1-xFe₂O₄. This could be explained using the cation distribution, which can be written as, (Co_0.28-0.28xFe_0.72+0.28x)[Ni_xCo_0.72-0.72xFe_1.28-0.28x]O₄. The two values of the saturation magnetization and the coercivity decreased, as the rate of nickel substitution was increased. These decreases could be explained using the cation distribution, the magnetic moment, and the magneto crystalline anisotropy constant of the substituted ions.