The　inherent　ductile-to-brittle　transition　(DBT)　of　body-centered　tetragonal　Sn　at　cryogenic　temperatures　restricts　the　use　of　Sn-based　solders　in　the　interconnection　of　cryogenic　electronics,　but　little　is　known　about　the　deformation　behaviors　accompanying　with　the　transition　and　the　underlying　transition　mechanism.　In　this　work,　the　deformation　features　before　cryogenic　brittle　fracture　and　the　DBT　mechanism　in　polycrystalline　Sn　were　studied　through　uniaxial　tensile　experiments　at　different　temperatures.　Compared　to　the　softening　process　stimulated　by　dynamic　recovery　and　dynamic　recrystallization　before　ductile　fracture　at　room　temperature　(similar　to　293　K),　a　high　strain　hardening　rate　(similar　to　5%　of　the　shear　modulus)　is　maintained　during　the　linear　hardening　period　preceding　brittle　fracture　at　the　liquid　nitrogen　temperature　(similar　to　77　K)　due　to　the　pronounced　intersecting　of　{301}　deformation　twins.　But　the　irreconcilable　velocity　difference　between　dislocation　glide　(similar　to　3　mu　m/s)　and　twin　thickening　(　10　mu　m/s)　at　77　K　leads　to　a　premature　brittle　fracture　in　the　midst　of　the　linear　hardening,　and　indeed　the　DBT.　The　suggested　specific　DBT　mechanism　is　substantiated　by　the　fact　that　a　significant　increase　in　the　velocity　(similar　to　1500　mu　m/s)　with　the　increasing　temperature　(123　K)　allows　the　dislocation　slip　to　readily　accommodate　the　shear　strains　due　to　{301}　twin　thickening　at　the　grain　boundaries,　thereby　resulting　in　ductile　fracture　rather　than　brittle　fracture.　This　deep　understanding　about　the　DBT　in　polycrystalline　Sn　may　help　forge　a　new　path　to　design　ductile　and　strong　Sn-based　solders　and　solder　joints　for　cryogenic　electronics　by　deformation　twinning.　(C)　2022　Elsevier　B.V.　All　rights　reserved.