The dependence of optical characteristics on the structure of atomic-layer-deposited titania (TiO2) thin films is studied. Amorphous films grown at 100 degrees C have an optical band gap of 3.3 eV while the refractive index values of these films range from 2.2 to 2.3 at a wavelength of 633 nm. The refractive index can be increased up to 2.65 by using the growth temperatures about 300 degrees C. Composition and structure studies reveal that the formation of preferentially oriented crystal (anatase) structure contributes to this increase of refractive index most significantly.