TiO2 films deposited on well clean K9 glass substrates are investigated for their thickness and optical constants by transmittance and X-ray reflectivity (XRR) measurements. XRR provides highly accurate results on film electron density, thickness and interface roughness. The film thickness from XRR is used as an anitial value to accelerate the evaluation of the transmittance data. With the help of Forouhi-Bloomer dispersion model, the calculated transmittance curves fit the experimental results well. For the same sample, the film thickness values from the two methods are very close and the maximum difference is 4.9 nm, which indicates that the combination of XRR and transmittance spectra can improve the reliability of the optical characterization.