Revealing　the　mesoscale　deformation　mechanisms　of　titanium　alloy　with　tri-modal　microstructure　is　of　great　significance　to　improve　its　mechanical　properties.　In　this　work,　the　collective　behavior　and　mechanisms　of　slip　activities,　slip　transfer,　and　grain　boundary　sliding　of　tri-modal　microstructure　were　investigated　by　the　combination　of　quasi-in-situ　tensile　test,　SEM,　EBSD　and　quantitative　slip　trace　analyses.　It　is　found　that　the　slip　behavior　presents　different　characteristics　in　the　equiaxed　alpha　(　alpha　p　)　and　lamellar　alpha　(　alpha　l　)　grains.　Under　a　low　level　of　deformation,　almost　all　the　slip　deformation　is　governed　by　single　basal　and　prismatic　slips　for　both　of　alpha　p　and　alpha　l　,　despite　small　amount　of　＜　a　＞　-pyramidal　slip　exists　in　alpha　l　grains.　As　deformation　proceeds,　＜　a　＞　-pyramidal　and　＜　c　+　a　＞　-pyramidal　slip　systems　with　high　Schmid　factors　were　activated　in　quantities.　Specially,　certain　coarse　prismatic　slip　bands　were　produced　across　both　of　single　and　colony　alpha　l　grains　whose　major　axes　tilting　about　40　degrees-70　degrees　from　the　tensile　axis.　Slip　transfer　occurs　at　the　boundaries　of　alpha　p　/　alpha　p　and　alpha　l　/　,B　under　the　condition　that　there　exists　perfect　alignment　between　two　slip　systems　and　high　Schmid　factors　of　outgoing　slip　system.　The　slip　transfer　across　alpha　l　/　,B　boundary　can　be　divided　into　two　types:　straight　slip　transfer　and　deflect　slip　transfer　with　a　deviation　angle　of　5　degrees-12　degrees,　depending　on　the　alignment　of　slip　planes　of　two　slip　systems.　The　grain　boundary　sliding　along　boundaries　of　alpha　l　/　,B　and　alpha　p　/　,B　was　captured　by　covering　micro-grid　on　tensile　sample.　It　is　found　that　the　crystallographic　orientation　and　the　geometrical　orientation　related　to　loading　axis　play　great　roles　in　the　occurrence　of　grain　boundary　sliding.　(c)　2022　Published　by　Elsevier　Ltd　on　behalf　of　The　editorial　office　of　Journal　of　Materials　Science　&　Technology.