Nanoparticles of MgO were successfully prepared in Triton X-100/n-hextnol/cyclohexane/water W/O microemulsion system. Basic preparation conditions were determined. TG-DTA showed that the proper calcination temperature was 450 oC. The nanoparticles were characterized by XRD and IR measurements in order to determine the crystal phase and component of the products. The morphological feature of the nanoparticles has been investigated by TEM. The result of XRD pattern demonstrated that the particles were exactly indexed to a cubic rock salt structure of MgO. There are no organic or inorganic impurities appeared after 450 oC calcination. The result of TEM showed that the MgO nanoparticles were spherical and uniform. The UV-visible spectra demonstrated the phenomenon of absorption blue shift, which is the evidence of quantum confinement effect, with the decrease of water/surfactant(R) value. Average size of the nanoparticles increased with increasing R value.