In this work, the synthesis of Ni1−x Mg x Fe2O4 (0 ≤ x ≤ 1.0) nanoparticles by a facile microwave-assisted combustion method is reported with detailed study of the structural and optical properties. By employing techniques of x-ray diffraction (XRD) with Rietveld refinement, transmission electron microscope (TEM, HRTEM, TEM-EDX), Fourier transform infrared spectroscopy (FTIR) and UV-Visible spectroscopy, the synthesized nanoparticles are characterized and introduced for further study of size-confined properties. Nanocrystals of a pure cubic spinel structured phase with average particle size of 20–40 nm were successfully synthesized in the whole range of x. In consistence with Vegard's law for a solid solution lattice, the lattice constant increases linearly with the substitution for Ni2+ with the relatively larger Mg2+ cations. The Rietveld analysis of the observed XRD patterns reveals an inversed spinel structure in NiFe2O4 nanoparticles with a decreased inversion factor by Mg-substitution. The results of UV-Visible absorbance indicate a wide energy gap of about 3.6 eV for NiFe2O4 nanoparticles thatmonotonically tuned towards a narrow band gap by Mg-doping.