HfO2 thin films were prepared by radio frequency (RF) magnetron sputtering at different RF powers. The influence of RF power on optical properties of HfO2 thin films were investigated by spectroscopic ellipsometry (SE) toghther with high-resolution transmission electron microscopy (HR-TEM) and Fourier transform infrared spectroscopy (FTIR). The results show that there is a SiO2 interface layer between HfO2 thin film and Si substrate. A four layer structured model consisting of SiO2 interfacial layer was used to fit the SE data. With the increasing RF power, the refractive index of the HfO2 thin films increases firstly and then decreases and, the extinction coefficient of the HfO2 thin films increases little.