Nanocrystalline Co-substituted Ni–Zn ferrites have been synthesized by employing a co-precipitation technique using oxalate precursors. The co-precipitated oxalates were decomposed in air at 650 °C for 1 h and the powder was sintered at 1000 °C. The phase formation of the sintered ferrite was confirmed by x-ray diffraction studies. The lattice parameter a (Å) increases with the addition of Co²⁺ which is attributed to the large ionic size of Co²⁺ (0.78 Å), which replaces Ni²⁺ (0.74 Å). The crystallite size has been estimated by the Debye–Scherrer formula using the full width at half-maximum (FWHM) of the line broadening of the (311) reflection and it is found to be in the range of 25–28 nm. Also, with increasing cobalt content the magnetocrystalline anisotropy constant (K₁) and magnetostriction constant (λ_s) increase, and the grain size decreases, hence the value of initial permeability μ_i decreases. The saturation magnetization M_s and magnetic moment (n_B) do not show any significant variation for all compositions varying with the cobalt content.