Nanostructured NiO samples having different average particle sizes were prepared and the variations of the real and imaginary components of the complex dielectric function epsilon* were studied as a function of the frequency of the applied signal and temperature. The dielectric relaxation mechanism is discussed considering nanostructured NiO as a carrier dominated dielectric with high density of hopping charge carriers. The observed omega(n-1) dependence of the real and imaginary components of e* is discussed in the light of the 'Universal' model of dielectric response. The various contributions to the measured dielectric loss epsilon" such as the steady state charge transport, delayed readjustment of screening charges and the Debye delays are discussed. It is shown that the temperature dependence of both the real and imaginary components of e* are in accordance with the models used for discussing the dielectric relaxation and loss mechanisms. The variation of epsilon' with average particle size seems to be rather complex depending on a number of parameters associated with the interfacial region which vary with the average particle size. (C) 2003 Kluwer Academic Publishers.