The paper presents a complex study of the effect of Mg substitution on the functional properties of Ni-ferrite ceramics prepared by self-combustion sol-gel method. The sintered ceramics have pure cubic spinel structures with an increase of the lattice parameter and the grain size with Mg content. The electrical properties of Mg (x) Ni1-x Fe2O4 (x = 0; 0.17; 0.34; 0.5; 0.66; 1) ceramics have been investigated. The complex impedance spectra suggest a grain boundary contribution in the conduction process and reveal that the real part of impedance and the imaginary component (reactance) increase with increasing the Mg amount. The temperature dependence of dielectric properties shows that the hopping of charge carriers is thermally activated. The resistivity as a function of frequency for different degree of humidity was also investigated. All the investigated samples show a typical ferrimagnetic character with a strong non-linearity, small coercitive field (similar to 50 Oe) and a saturation field of similar to 1kOe, typical to the investigated Mg (x) Ni1-x Fe2O4 ceramics. The Curie temperature determined from magnetic susceptibility versus temperature dependences presents a decrease with the addition of non-magnetic Mg2+ ion concentration from 603 A degrees C (for x = 0) to 384 A degrees C (for x = 1).