LaF3 single layers are prepared by thermal evaporation at different deposition temperatures on JGS1. Refractive index and extinction coefficient are obtained from the measured transmittance and reflectance spectral curve. Atomic force microscopy (AFM) is used to measure the surface morphology and roughness. The crystal structures and grain size of the thin films are evaluated by X-ray diffraction (XRD). It is found that the crystallization status becomes more compact and the refractive index increases with the deposited temperature. The optical constants and refractive index inhomogeneity of the thin films present linearity. The increasing total optical loss with deposited temperature is attributed to the absorption because the scattering occupies a very low percent in the whole loss.