CuO nanoparticles were synthesized by TritonX-100 based water-in-oil reverse micelles,system, and characterized by X-ray diffractometer (XRD) and transmission electron microscopy (TEM), respectively. The results indicated that the grain size, morphology and distribution were highly dependent on the ratio of water to surfactant (R). The control effect of water content dissolved in reverse micelles was attributed to the enhancement of the exchange rate of reactants among micelles and the relevant change in the interfacial film flexibility of reverse micelles with the increasing amount of water in the microemulsion. (C) 2007 Elsevier B.V All rights reserved.