Strengthening-toughening　mechanism　of　powder　metallurgy　Ti-Si　alloy　was　investigated　by　interrupted　in-situ　tensile　tests.　The　fabricated　Ti-Si　alloy　was　strong　and　ductile.　Slip　band　was　blocked　mainly　by　solutes　and　grain　boundaries.　Narrow　deformed　strips,　aligned　low　angle　grain　boundaries　and　twin　boundaries　also　played　a　positive　role　in　this　blockage.　Cross-slip　in　the　strain　hardening　stage　lowered　the　difference　value　of　ultimate　tensile　and　yielding　strength.　Two-stage　toughening　mechanism　was　studied　(necking　onset　was　demarcation　point).　In　the　homogeneous　deformation　period　before　necking,　strain　compatibility　was　improved　by　the　intergranular　rotation　(rotation　axis　along　[0001],　[-1100]　and　[-1102]),　high-angle　intragranular　rotation,　intergranular　sliding　and　minor　twinning　deformation.　In　the　heterogeneous　deformation　period　after　necking,　microcracks　initiation/coalescence　was　dragged　by　uniform　stress　distribution.　Cracks　propagation　was　also　suppressed　by　dense　grain　boundaries　and　front　plastic　grains.　The　present　comprehensive　analysis　of　strength　and　toughness　provided　an　in-depth　understanding　about　the　relationship　among　the　processing,　mechanical　properties　and　microstructure　of　Ti　alloy.　(C)　2016　Elsevier　B.V.　All　rights　reserved.