Abstract: The crack and other defects of girth weld of an in-service X70 grade steel gas pipeline were analyzed by means of nondestructive test, weld anatomical analysis, physical and chemical performance test, macroscopic and microscopic analysis, etc. The results show that there are 5 defects such as slag inclusions, shrinkage holes and incomplete fusions at the 1-2 o'clock position of the weld, which decreases the strength and toughness of the weld. From 11 to 12 o'clock, there is a crack with a length about 20mm. The crack cracks along the fusion line of the root weld, expands along the coarse-grained zone, and penetrates the backing weld. Big weld misalignment and discontinuous shape at the root of the weld lead to stress concentration.The reheat crack is formed under the stress concentration and thermal welding stress during the heating to the backing weld by subsequent filling welding.And it does't expand during service.