A　gradient　nanostructured　surface　layer　was　fabricated　in　the　commercial-purity　titanium　(Ti)　thin-wall　tubular　sample　using　the　modified　surface　nanocrystallization　(SNC)　technique.　Biaxial　tension-torsion　fatigue　behavior　of　the　SNC　Ti　was　investigated.　The　SNC　Ti　shows　significant　longer　biaxial　fatigue　lives　than　the　coarse　grained　Ti　(CG　Ti)　at　the　same　cyclic　equivalent　stress　amplitude.　Both　CG　and　SNC　Ti　display　hardening　during　cyclic　deformation,　and　the　hardening　level　in　the　SNC　Ti　is　larger　than　that　of　the　CG　one.　Microstructural　analysis　reveals　that　the　SNC　Ti　shows　hierarchical　deformation　mechanisms　in　different　areas　across　the　wall-thickness　of　tubular　samples　during　biaxial　fatigue.　In　the　nano/ultrafine　grain　region,　the　stress-driven　nanograin　growth　is　the　primary　deformation　mechanism.　In　the　deformed　grain　region,　the　interaction　between　lamella　structure　and　dislocations　is　observed.　In　the　coarse　grain　region,　prismatic　slip　is　main　deformation　mode.　The　initiation　of　fatigue　cracks　is　restricted　in　the　SNC　Ti　since　the　stress　concentration　is　relieved　through　stress-driven　nanograin　growth　during　cyclic　deformation.　The　initiation　mechanism　of　fatigue　cracks　transforms　from　slip　band　and　grain　boundary　cracking　in　the　CG　Ti　to　the　shear　band　cracking　in　the　SNC　Ti.　Furthermore,　since　the　superposition　of　residual　compressive　stress　with　the　exterior　applied　stress　in　the　SNC　Ti,　both　maximum　shear　stress　and　maximum　normal　stress　are　decreased.　Consequently,　the　equivalent　driving　forces　for　crack　initiation　and　propagation　in　the　SNC　Ti　are　smaller　than　those　in　the　CG　Ti.　The　higher　resistance　to　fatigue　crack　initiation　and　the　lower　equivalent　driving　force　for　fatigue　crack　initiation　and　propagation　contribute　to　the　enhancement　of　fatigue　properties　in　the　SNC　Ti.