Monophasic nanocrystals of rare earth gadolinium (Gd 3+) ion doped nickel-cobalt ferrite, Ni 0.5 Co 0.5 Gd y Fe 2−y O 4 (y = 0.00, 0.025, 0.050, 0.075, 0.100 and 0.125) have been synthesized by l-ascorbic acid assisted sol-gel auto-ignition process. Thermogravimetric and differential scanning calorimetry, X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) with selected area electron diffraction (SAED) patterns, ultraviolet-visible spectrophotometry and vibrating sample magnetometer were employed to characterize the microstructure, optical and magnetic properties. Structural investigations verified the presence of cubic spinel phase short of any contamination peak. The average crystallite size was seen in nano-regime dimensions and well confirmed by TEM results. SAED patterns showed bright and intense ring of (311) plane revealed the establishment of spinel ferrite nanocrystals. Lattice constant (a) increases steadily with the rise in Gd 3+ content following the Vegard's law. FTIR spectra confirmed the creation of spinel ferrite structural geometry. Saturation magnetization (M s) and a remanence magnetization (M r) decreased from 29.77 to 20.78 emu/g and 21.62 to 15.79 emu/g, respectively, with increasing the Gd 3+ content. However, coercivity (H c) is significantly increased. The highest value of H c = 1132 Oe was found for y = 0.075 sample. The reduction in M s values can be justified owing to the replacement of Fe 3+ existing at the octahedral [B]-sites by the Gd 3+ ions which is paramagnetic at room temperature. The elucidation of magnetic parameters with Gd 3+ doping was attributed to the discrepancy of anisotropy constant, the paramagnetic nature of Gd 3+ and crystallite size effect.