ZnO films have been deposited on oxide grown Si wafers by the conventional thermal evaporation method. Anhydrous zinc acetate was directly heated and sublimed in the laboratory-made brass boat. The substrates temperature varied from $200^{circ}C$ to $600^{circ}C$. Oxygen has been flowed into the deposition chamber to change the partial pressure of oxygen. The films deposited at high oxygen pressure exhibited higher resistivity than films at low pressure. X-Ray Diffraction(XRD), Energy Dispersive Spectroscopy(EDS) and Rutherford Backscattering Spectrometry (RBS) were conducted on the films to reveal the crystallinity and composition of the ZnO films. The ZnO films deposited at high oxygen pressure were extremly sensitive to the humidity of higher than 70 % RH.