Using　first-principles　density　functional　calculations,　we　have　investigated　the　electronic　structures　of　Ti-doped　ZnO　(Ti　is　in　4　oxidation　state)　with　and　without　oxygen　vacancy.　The　ferromagnetic　property　is　identified　in　the　presence　of　oxygen　vacancy　despite　Ti　being　nonmagnetic　in　its　natural　phase.　The　ferromagnetism　originates　from　the　charge　transferring　from　donor　derived-defect　band　to　unoccupied　Ti-3d　states　and　the　hybridization　between　Ti-3d　and　O-2p　band　near　the　Fermi　level.　On　increasing　the　oxygen　vacancy　concentration,　a　transition　from　a　long-ranged　magnetic　order　to　a　short-ranged　interaction　is　found　and　the　oxygen　vacancies　prefer　to　distribute　non-uniformly　in　Ti-doped　ZnO.　©　2011　Elsevier　B.V.　All　rights　reserved.