Abstract: Ultra fine copper oxide powder was prepared employing precipitation on the interfaces of microemulsions with P204 diluted by carbon tetrachloride as an extractant, and oxalic acid aqueous solution as a back extractant and precipitator. The effect of preparation conditions on the structure and size distribution of copper oxide powders is systematically discussed. The results show that the extraction conditions play an important role in the structure and size distribution of copper oxide powders. A high ratio of P204 to CCl₄ over 20% is favorable to the formation of particles with small size. During the process of back extraction, the time of extraction presents little influence on the powders. Under an optimum condition with the content of CuSO₄ aqueous solution around 0.10～0.12 mol/L and extraction temperature 15～20 ℃, black brown, well-dispersed copper oxide powders with an average size of less than 100 nm and high purity of monoclinic crystal phase were obtained by interfacial precipitation.