Twin　boundary　can　both　strengthen　and　soften　nanocrystalline　metals　and　has　been　an　important　path　for　improving　the　strength　and　ductility　of　nano　materials.　Here,　using　in-lab　developed　double-tilt　tensile　stage　in　the　transmission　electron　microscope,　the　atomic　scale　twin　boundary　shearing　process　was　in　situ　observed　in　a　twin-structured　nanocrystalline　Pt.　It　was　revealed　that　the　twin　boundary　shear　was　resulted　from　partial　dislocation　emissions　on　the　intersected　{111}　planes,　which　accommodate　as　large　as　47%　shear　strain.　It　is　uncovered　that　the　partial　dislocations　nucleated　and　glided　on　the　two　intersecting　{111}　slip　planes　lead　to　a　transition　of　the　original　＜　110　＞　symmetric　tilt　Sigma　3/(111)　coherent　twin　boundary　into　a　＜　110　＞　symmetric　tilt　Sigma　9/(114)　high　angle　grain　boundary.　These　results　provide　insight　of　twin　boundary　strengthening　mechanisms　for　accommodating　plasticity　strains　in　nanocrystalline　metals.