Nanoparticles of NiO were successfully prepared by Triton X-100/n-hextnol/cyclohexane/water W/O microemulsion system. Basic synthesis parameters were determined. The result of TG-DTA analysis of the precursor products showed that the proper calcination temperature was 450 degreesC. The nanoparticles were characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM) and UV-VIS spectra. The results showed that the nanoparticle was exactly NiO with cubic rock salt structure and the size of nanoparticles was strongly affected by the ratio of water to surfactant (R) and also the calcined temperature. With the R value increased, the particles size became larger; with the calcined temperature increased, the particles size also became larger. (C) 2004 Elsevier B.V. All rights reserved.