Effects of Ta doping on microstructure, surface morphology, band gap and optical properties of HfO2 thin films deposited by reactive RF magnetron sputtering have been investigated by X-ray diffraction （XRD）, atomic force microscopy （AFM） and spectrophotometry. The results indicate that the introduction of Ta can improve the film crystallization temperature to 800,900 and 950℃ for HfTaO thin films containing 10 % Ta, 26 % Ta and 50%Ta,respectively. When the ratio of Ta/Hf＋Ta is about 72% ,the film remains amorphous at a high temperature up to 950℃. Thermal stability of the amorphous HfTaO film was significantly improved by the controlled addition of Ta. AFM results show that the amorphous HfTaO film is smooth. Refractive indice of the films increases from 1.90 to 2.15 with increasing Ta percentage. Eg was determined to be in the range 4.15-5.29 eV,the band gap energy of the HfTaO films as a function of increasing Ta percentage decreases.