Abstract: The microstructure and mechanical properties of X100 pipeline steel were investigated by optical electron microscope, scanning electron microscope and material testing machine.The results indicate that the microstructure of X100 line pipe is mainly composed of acicular ferrite and granular bainite in weld zones.The polygon ferrite exists in HAZ and the grains are coarser,which lead to softening and embrittlement.The fusion line is clear between outer weld and inner weld.The tensile strength and elongation rate of the welded joint are up to the 805 MPa and 10.7%,respectively.The impact energy is more than 110 J,and the average percent of shear fracture gets to 85% at - 10℃,which is ductile fracture.The results of hardness test show that the hardness of the HAZ of outer weld is higher than that of inner weld.