Atomic layer deposition of TiO 2 thin films on single crystal α-Al 2 O 3 (1102) (sapphire) substrates was studied and compared with that on Si(100) and amorphous SiO 2 . The TiCl 4 -H 2 O process allowed epitaxial growth of rutile on α-Al 2 O 3 (1102) at 425 o C and higher temperatures. In the epitaxial films, the (101) plane of rutile was parallel to the (1102) plane of α-Al 2 O 3 and the in-plane orientational relationship was determined to be [101]rutile [1101]α-Al 2 O 3 . Although no epitaxial films were obtained at lower temperatures, the refractive indices of films deposited on α-Al 2 O 3 (1102) substrates were higher than the corresponding values of crystalline films grown on SiO 2 under similar conditions. The refractive index values measured at the wavelength of 580nm were 2.4-2.7 and 2.80-2.82 in the case of non-epitaxial and epitaxial films, respectively, grown on α-Al 2 O 3 . The band gap of epitaxial rutile was determined to be 3.16eV.