Copper substituted Ni0.7Co0.3-xCuxFe2O4 (x=0.0, 0.05, 0.1, 0.15 and 0.2) nanocrystalline ferrites are synthesized by the sol-gel method. The x-ray powder diffraction (XRD) patterns show that all samples have a pure single-phase cubic spinel structure with Fd-3m space group over the whole composition range. From XRD patterns, the lattice constants, average crystallite size, grain size of these compounds have also been calculated and compared to nickel ferrite. Most of the values are found to be increased with increasing copper concentration.