Nanometer MgO were prepared by sol-gel auto combustion techniques using magne-sium nitrate as raw material and citric acid as chelating agent. IR spectra of dried gels were examined to investigate the structural of precursors. By studying on the different TG curves of magnesium citrate gel, which prepared by different methods, we found that combustion process occurred and the nitrate ions acted as oxidant in combustion process. TEM photographs of synthesis powders by sol-gel auto-combustion showed that the crystallites were uniform. XRD curves of this sample showed that the particle sizes are 8.9 nm. The BET curves showed that the specific surface of the sample is 26.34 m2/g. And mechanism of foaming process in restraining agglomeration were discussed.