Abstract: Cracks were found on the surface of tee of a natural gas pipeline during regular magnetic particle inspection. The cracking causes of tee were analyzed by means of chemical composition analysis, mechanical property test, hardness test, macro analysis, fracture analysis, metallographic examination and energy spectrum analysis. The results show that the metal oxide of the tee for the natural gas pipeline could not be squeezed out during the extrusion process of high frequency welding of straight seam steel pipe. The distribution of long strip oxide inclusions along the fusion line reduced the bonding strength of the base metal, and finally led to the stress cracking of the tee in service.