Abstract: The welded joint of a longitudinally submerged arc welded (LSAW) pipeline of X80 steel at the job site of the second project of natural gas transmission from west to east China was studied by microstructure observation,hardness testing,uniaxial tensile testing at room temperature.The results show that welding hardening happened near the fusion zone of the welded joint and welding softening happened at the fine-grained heat affected zone.The grains of granular bainite near the fusion zone coarsened severely and contained large hard-brittle inclusions of M/A constituents,meanwhile mutual permeation between the microstructure on both sides of the fusion line happened,so the fusion zone was the fracture risk zone of the welded joint.The welded joint of X80 steel was overmatched,and the base metal had a good capacity of deformation strengthening.The weld metal had poor plasticity and toughness because of widmanstatten ferrite (WF),with the ratio of yield strength to tensile strength of 93.55% and breaking elongation of 14.2%.The heat affected zone overall possessed relatively high strength and plasticity,and good mechanical properties with tensile strength of 669.30 MPa,the ratio of yield strength to tensile strength of 89.45% and breaking elongation of 22.8%.