By　in　situ　uniaxial-tensile　tests　with　dynamic　and　atomic　scale　observation,　we　report　the　quantitative　investigation　of　strain-induced　deformation-twinning　process　through　incoherent　twin　boundary　propagation　in　nanocrystalline　Au.　The　consecutive　and　quantitative　strain　maps　revealed　that　a　strong　compressive　strain,　up　to　5.8%,　was　formed.　A　screw-rotation　twinning　mechanism　by　consecutive　and　collective　screw-rotations　of　triple-partials　along　a　[111]　screw-axis　is　proposed.　This　twin　generates　a　macro-strain　of　0.707,　same　as　the　conventional　shear　twins,　but　in　a　helical　manner.　(C)　2012　American　Institute　of　Physics.　[http://dx.doi.org/10.1063/1.4752747]