In-situ TEM tension was used to observe the deformation of bulk pure Al and that constrained by bi-interface.The results show that cavity is joined with principal crack just after forming in front of crack tip in bulk Al.Different from bulk Al,however,more cavities will generate in front of the cavity in constrained Al sheet by bi-interface before main crack propagates,that is cavitations and blunting and sharpening of main crack coexist in tension process.Dislocations emitted from crack tip pile up more remarkably at the grain boundaries around crack tip with decrease of thickness of pure Al sheet resulting in obvious inerease of stress concentration,which leads to brittleness of pure Al sheet.