To study the effect of α phase morphology(equiaxed α(αE)/lamellar α(αL))on the in situ tensile behavior of TC21 alloy,the slip band,dislocation,crack initiation,and propagation were analyzed by scanning electron microscopy(SEM)with in situ tensile stage and transmission electron microscopy(TEM).The results show that the slip bands first concentrate in the αE phases and easily truncate at the α/β phase boundaries,whereas the slip bands move across the α lamellae in the αL phase.Microcracks are easily generated in αE or α/β phase boundaries with large plastic deformation.When the quantity of αL is more than αE,the crack tip is more easily deflected at α phases or α clusters with different orienta-tions,making the main crack propagation path more zig-zag.When the volume fraction ratio of αE to αL is～3:4,i.e.,the volume fraction of αE is close to that of αL,TC21 alloy exhibits better strength and slower crack propagation rate.