Near stoichiometric ratio lithium tantalate (LiTaO 3) thin film possesses unique performance advantages compared with bulk LiTaO 3 single crystal in applying in thermal sensors and detectors. Here, LiTaO 3 thin film is prepared on Pt/Ti/SiO 2/Si (100) substrate by the method of RF magnetron sputtering. The authors conduct research on the structure and optical properties of LiTaO 3 thin film under the conditions of annealing temperatures varying from 550 C to 700 C. Besides, the authors also study on optical constants of the LiTaO 3 thin film which includes refractive indices, extinction coefficients and optical band gap energy by spectroscopic ellipsometry. The results show that optical band gap energy E g will increase along with the grain size's increase. Moreover, the results demonstrate that the best annealing temperature is 600 C. Under the best annealing condition, the obtained refractive index, extinction coefficient k and optical band gap energy E g is 2.05, 2× 10− 5 and 3.82 eV, respectively. These values are consistent with that of lithium tantalate single crystal. All results above mentioned proving that lithium tantalate (LiTaO 3) thin film prepared by RF magnetron sputtering possesses a good crystallinity and ordered structure.