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.