Un-doped ZnO films were deposited by direct current (DC) magnetron sputtering and the film structure and properties were characterized by X-ray diffraction(XRD), UV/VIS spectrometer and atomic force microscope(AFM), respectively. The effects of process parameters, such as oxygen partial pressure, on the properties of ZnO film were studied. It is shown that, the average transmittance in the visible region can reach 88% and it increases with higher oxygen partial pressure, and the root mean square (RMS) roughness is about 0.41nm. For the solar cells with the structure of glass/ITO/ CdS/CdTe, un-doped ZnO films are deposited as the buffer layer between ITO layer and CdS layer, then the open voltage and filling factor of the cells are greatly improved. The conversion efficiency of the devices with ZnO films reaches up to 13.6%.