The　effect　of　surface　nanocrystallization　(SNC)　on　torsion　fatigue　property　of　pure　titanium　was　investigated.　Compared　with　coarse-grained　Ti　(CG　Ti),　the　SNC　Ti　exhibits　the　longer　fatigue　life　at　the　same　cyclic　stress　amplitude　and　the　higher　fatigue　strength　limit.　Both　CG　and　SNC　Ti　display　cyclic　softening　during　torsional　loading.　The　cyclic　softening　rate　of　SNC　Ti　is　lower　than　that　of　CG　Ti.　Microstructural　analysis　reveals　that,　fatigue　crack　initiation　is　suppressed　in　the　SNC　Ti　since　the　surface　gradient　nano/ultrafine　grained　layer　restrains　strain　localization.　As　a　result,　the　mechanism　of　crack　initiation　changes　from　grain　boundary　and　twin　boundary　cracking　to　shear　band　cracking.　Parallel　dislocation　lines,　embryonic　cells　and　twins　are　observed　in　the　fatigued　CG　Ti.　In　comparison,　no　obvious　variation　of　substructures　can　be　distinguished　in　the　SNC　Ti　before　and　after　torsion　fatigue.　The　relative　stable　microstructure　in　the　SNC　Ti　facilitates　the　improvement　of　fatigue　life.　Furthermore,　the　large　residual　compressive　stress　on　the　surface　restrains　the　fatigue　crack　propagation　in　the　SNC　metals.　Therefore,　the　SNC　Ti　displays　the　better　torsion　fatigue　properties　than　the　CG　Ti.　(C)　2015　Elsevier　B.V.　All　rights　reserved.