Understanding the properties of semiconductor nanostructures is important for developing their practical applications in the nanodevices. Although the electrical properties of spinel oxides containing Ni and Co have been studied extensively over the last decades, there is still a significant disagreement on their electrical conduction mechanisms and the distribution of the various charge states of Co and Ni in the octahedral and tetrahedral sites of the spinel crystal lattice. In this study, Co3O4, NiO, and mixed Ni-Co oxide nanoparticles (NPs) of ∼20 nm are synthesized by co-precipitation method. The mechanism of the electrical conduction in the Co3O4 and Ni-Co oxides NPs is due to the hopping process in the octahedral site or between the octahedral and tetrahedral sites while in the NiO NPs, the large polarons conduction mechanism dominates. The thermoelectric power measurements confirm that the Ni-Co oxides NPs are non-degenerate semiconductors and the diffusion mechanism is the dominant component of the Seebeck coefficient. In contrast, the Co3O4 NPs show degenerate features with Fermi energy 0.054 eV. The NiO NPs exhibit a transition from non-degenerate to degenerate state. The magnetic measurements reveal an antiferromagnetic-paramagnetic transition at Néel transition temperature. The results are helpful to understanding the fundamental characteristics of the NPs under study.