DC conductivity of consolidated nanoparticles of NiO, having different average particle sizes (2.5nm-17nm) was measured in the temperature range 313 K to 423 K, The conductivity of NiO nanoparticles are found to be enhanced by six to eight orders of magnitude over that of NiO single crystals. This large enhancement in conductivity is attributed to the high density of Ni2+ vacancy in the nanoparticles. Calculation of activation energy, leads to the conclusion that the most prominent conductivity mechanism over the temperature range of observation is the large polaron conduction associated with holes in the 2p band of O2-. The decrease in activation energy for the samples in comparison with that of the bulk material is explained on the basis of the enormously defective nature of nanoparticles, In the last part of the discussion, the effect of interfacial region on the electrical conductivity of the samples is analyzed, by taking into account the contributions due to grain boundaries and triple junctions. A semi-quantitative explanation for the observed variation of conductivity with particle size is presented on the basis of the assumption that the role of the triple junctions is to reduce the conductivity, The importance of triple junctions in determining the transport properties of nanoparticles is emphasized.