β　titanium　alloys　with　bi-modal　structure　which　exhibit　improved　strength-ductility　combination　and　fatigue　property　are　widely　used　in　aviation　and　aerospace　industry.　However,　owing　to　the　small　size　of　primary　α　(αp)　and　nano-scaled　multi　variant　distribution　of　secondary　α　platelets　(αs),　investigating　the　deformation　behavior　is　really　a　challenging　work.　In　this　work,　by　applying　transmission　electron　microscopy　(TEM),　the　slip　behavior　in　αp　and　transformed　β　matrix　with　different　tensile　strain　was　studied.　After　α/β　solution　treatment,　the　initial　dislocation　slips　on　{110}　plane　with　direction　in　β　matrix.　During　further　deformation,　(110),　(101)　and　(1　1¯　2)　multi　slip　is　generated　which　shows　a　long　straight　crossing　configuration.　Dislocation　cell　is　exhibited　in　β　matrix　at　tensile　strain　above　20　%.　Different　from　the　solid　solution　treated　sample,　high　density　wavy　dislocations　are　generated　in　transformed　β　matrix.　High　fraction　fine　αs　hinders　dislocation　motion　in　β　matrix　effectively　which　in　turn　dominates　the　strength　of　the　alloy.　In　primary　α　phase　(αp),　a　core-shell　structure　is　formed　during　deformation.　Both　pyramidal　a　+　c　slip　and　prismatic/basal　a　slip　are　generated　in　the　shell　layer.　In　core　region,　plastic　deformation　is　governed　by　prismatic/basal　a　slip.　Formation　of　the　core-shell　structure　is　the　physical　origin　of　the　improved　ductility.　On　one　hand,　the　work　hardening　layer　(shell)　improves　the　strength　of　αp,　which　could　deform　compatibly　with　the　hard　transformed　β　matrix.　Meanwhile,　the　center　area　(core)　deforms　homogeneously　which　will　sustain　plastic　strain　effectively　and　increase　the　ductility.　This　paper　studies　the　slip　behavior　and　reveals　the　origin　of　the　improved　strength-ductility　combination　in　Bi-modal　structure　on　a　microscopic　way,　which　will　give　theoretical　advises　for　developing　the　next　generation　high　strength　β　titanium　alloys.　©　2020