Copper oxide, a direct band gap semiconductor with band gap about 1.21-1.51 eV, has been regarded as a promising material for photovoltaic. Nanocrystalline copper oxide films have been synthesized on Si by dc sputtering method. The effects of oxygen flow rate and deposition temperature on the microstructure of nanocrystalline copper oxide films were investigated. X-ray diffraction analysis shows that a broaden peak of Cu2O (111) at 36.720 was observed at the deposition condition of DC power 150 W, pressure 2*10-2 Torr, substrate temperature 100 °C, Ar flow rate 15 sccm and O2 flow rate 1sccm. With increasing the oxygen flow rate to 3 and 5 sccm, CuO (-111) could be observed at 36.58o. The increase of oxygen flow rate resulted in the film formation from Cu2O to CuO. SEM pictures show that copper oxide films exhibit nanosize grains. X-ray diffraction patterns of CuO films deposited at 50~200 °C show that only (-111) plane is obtained. The SEM pictures show that the grain size increases with the deposition increases.