The effects of Ni²⁺ doping on the crystalline lattice and corresponding magnetic properties of Mg_1−xNi_xFe₂O₄ nanocrystals, synthesized by a microwave combustion, are systematically studied. Highly crystalline nanoparticles with average size of 20–30 nm and stoichiometric chemical compositions are indicated by results of transmission electron microscopy (TEM) and inductively coupled plasma mass spectrometry (ICP-MS), respectively. The structural properties including lattice parameter, cationic radii, cationic distribution, bond lengths and angles have been estimated based on Rietveld analysis of the x-ray diffraction (XRD) patterns. Mostly inverse spinel structure in MgFe₂O₄ with an observed nonmonotonic increase of the inversion factor by Ni substitution and almost complete inversion for × ≳ 0.5 indicate competitive site preferences of the cations. The synthesized nanocrystals exhibit soft single-domain