The　fracture　characteristics　of　laser　welded　BT20　titanium　alloy　joints　have　been　investigated　through　in-situ　tensile　tests　in　scanning　electron　microscope　using　a　single-edge　notched　specimen.　For　as-welded　specimens,　they　would　fracture　as　soon　as　the　crack　initiated.　The　specimen　after　post-weld　heat　treatment　experienced　a　long　metastable　stage　before　fracture.　Microcrack　initiated　along　slip　band　under　the　combination　action　of　shear　stress　and　normal　stress.　The　crack　would　deflect　when　it　came　across　slip　bands.　New　microcrack　would　initiate　at　the　position　of　slip　bands　intersecting　each　other　ahead　of　the　main　crack,　and　converged　with　the　main　crack　subsequently.　Post-weld　heat　treatment　can　improve　the　toughness　and　plasticity　of　laser　welded　joints　markedly.