A　series　of　Ti-doped　SnO　2(110)　surfaces　with　different　oxygen　vacancies　have　been　investigated　by　means　of　first　principles　DFT　calculations　combined　with　a　slab　model.　Three　kinds　of　defective　SnO　2(110)　surfaces　are　considered,　including　the　formations　of　bridging　oxygen　(O　b)　vacancy,　in-plane　oxygen　(O　i)　vacancy,　and　the　coexistence　of　O　b　and　O　i　vacancies.　Our　results　indicate　that　Ti　dopant　prefers　the　fivefold-coordinated　Sn　site　on　the　top　layer　for　the　surface　with　O　b　or　O　i　vacancy,　while　the　replacement　of　sublayer　Sn　atom　becomes　the　most　energetically　favorable　structure　if　the　O　b　and　O　i　vacancies　are　presented　simultaneously.　Based　on　analyzing　the　band　structure　of　the　most　stable　configuration,　the　presence　of　Ti　leads　to　the　variation　of　the　band　gap　state,　which　is　different　for　three　defective　SnO　2(110)　surfaces.　For　the　surface　with　O　b　or　O　i　vacancy,　the　component　of　the　defect　state　is　modified,　and　the　reaction　activity　of　the　corresponding　surface　is　enhanced.　Hence,　the　sensing　performance　of　SnO　2　may　be　improved　after　introducing　Ti　dopant.　However,　for　the　third　kind　of　reduced　surface　with　the　coexistence　of　O　b　and　O　i　vacancies,　the　sublayer　doping　has　little　influence　on　the　defect　state,　and　only　in　this　case,　the　Ti　doping　state　partly　appears　in　the　band　gap　of　SnO　2(110)　surface.　©　2012　Springer-Verlag.