The optical characterization of LaF3 thin film in DUV spectral range was experimental investigated by using a variable angle purged UV spectroscopic ellipsometer. In order to take into account the inhomogeneity, a theory model that dividing the single thin film into several sublayers was adopted. Two kinds of LaF3 thin films fabricated on fused silicate substrate with different substrates temperature were tested. From the obtained optical index and the physical thickness of different sublayer in the two different kinds of LaF3 thin films, it was found that, the inhomogeneity of the LaF3 thin film deposited with substrate temperature at 300°C was stronger than that of the LaF3 thin film deposited with substrate temperature at 250°C, indicating that the substrate temperature has important influence on the optical index and inhomogeneity of LaF3 thin films. For both of the two kinds LaF3 thin films, the agreement between the measured transmittance and the simulated transmittance using the parameters from regression of SE was nice, indicating that the selection of the material dispersion law and regression procedure were successful.