Three girth weld cracking events were found by X-ray inspection in one natural gas station. To determine the failure cause, one of the three failure events was studied by visual inspection, nondestructive testing, microstructure examination, scanning electronic microscopy (SEM) coupled with energy dispersive spectrometry (EDS), blasting tests, finite element simulations and a series physical and chemical tests. The results revealed that the welding defects were original failure factor, and these welding defects led to crack initiation and propagation through the wall thickness under three aspects of stress concentration effects with internal pressure. The stress concentration effects were originated from the unqualified inner chamfering angle, unequal wall thickness, and inherent shape of tee pipe. The reason causing welding defects of girth weld is that the slag inclusions were not cleaned up timely, and then the slag inclusions led to the existence of welding defects. Display omitted •The failure causes of girth weld cracking of tee pipe were determined.•The stress concentration effect was analyzed to determine failure cause.•The blasting tests was carried out to analyze the effect of welding process.•The influencing mechanism of welding defect on girth weld cracking was clarified.