The tensile mechanical properties of X80 pipeline steel base metal and its welded joint specimens were analyzed by tensile test, the fracture morphologies and chemical compositions were observed with SEM（scanning electron microscopy） and EDS（energy dispersive spectrometer）, and the fracture behaviors were investigated. The results show that elongation and section shrinkage of the base metal are greater than that of the welded joints; the tensile fracture of base metal is tough fracture, while the fracture of welded joint that produces stratification phenomenon is tough+brittle. The fiber area and dimple size that exist on fiber area in the base metal are greater than that in the welded joint， the morphology of radiation area is dimple, while the cleavage fracture appears in the welded joint, and the shear lip of base metal and welded joint are cleavage. Impurities that is contained in the welding joint are sulfide and oxide, which are an important factor to reduce mechanical properties of welded joint.