Abstract: The causes of cracks in girth weld of a X70 gas transmission pipeline with diameter of 660 mm were analyzed by means of geometric dimension inspection, radiographic testing, chemical composition analysis, mechanical properties testing, macroscopic and microscopic structure analysis. The results show that cracks appear near 6 o’clock of the girth weld.The lack of fusion at the weld toe of girth weld is the direct cause of cracks. Under the action of additional stress, the cracks propagate along holes and welding slag in root welding and filler welding metal.