Twinning　mechanism　of　powder　metallurgy　Ti-Si　alloy　was　investigated　by　interrupted　in-situ　tensile　tests.　Extension　twins　{10-12}　＜　10-1-1　＞　in　the　fine-grained　Ti-Si　alloy　were　found　in　the　uniform　deformation　period,　but　no　twinning　in　the　coarse　pure　Ti.　Three　deformation　twinning　nucleation　mechanisms　were　proposed:　i)　local　stress　concentration　by　neighboured　slip　incompatibility,　ii)　slip/twin　oriented　relationship　in　the　parent　grain　and　slip/twin　transfer　by　high　Luster-Morris　oriented　relationship.　The　interior　back-stress　state,　grains　rotation　and　dislocations　pile-up　drove　the　occurrence　of　detwinning　phenomenon.　Silicon　facilitation　twinning　verified　the　hypothesis　that　the　substitutional　Si　solutes　affected　the　core　structures　and　thus　the　mobility　of　screw　dislocations.　Enhanced　driving　force　and　decreased　energy　barrier　of　nucleation　in　the　micro/atomic　scale　were　further　proposed　in　the　twinning　activation.　It　was　expected　to　deepen　the　understanding　of　twinning/detwinning　behaviors　and　supply　direct　evidences　building　immature　twinning　mechanism.　In-depth　understanding　about　the　relationship　among　the　processing,　mechanical　properties　and　microstructure　of　Ti　alloy　was　facilitated　in　the　present　work.