The structural properties of HfO2 films could be improved by thermal treatment owing to their crystallization. We deposited HfO2 films on sapphire by radio frequency (RF) magnetron sputtering, whose base vacuum pressure was lower than 4.5 × 10?6 Pa, RF power was 100 W, working temperature was 200°C, working pressure was 3 mTorr, and the density of the active gas (Argon) was 20 sccm. After depositing the HfO2 films, the samples were thermally treated by rapid thermal annealing (RTA) in O2 ambient at different temperatures. Subsequently, the measured physical properties (structural, morphological, and optical) indicated that the crystallite size, refractive index at a wavelength of 632 nm, and packing density increased with rising temperatures. In particular, an HfO2 film thermally treated at 800°C in O2 ambient had the highest refractive index of 2.0237 and packing density of 0.9638. The relation between optical and structural properties was also analyzed.