Abstract: During the regular inspection of X80 pipe girth weld defects, a girth weld with several defects was found. In order to confirm the type of the defects and analyze the causes of the defects, the defective parts of girth weld were dissected and the mechanical properties of the girth weld were tested. The results show that the impact toughness of the girth weld at 0o’clock does not conform to the requirements, the mechanical properties of other positions meet the requirements. Bedsides,there are two kinds of defects in the girth weld, one is groove incomplete fusion at 1 : 30, the other is root crack at 6 : 20. The cause of groove incomplete fusion is that the slag inclusion produced during filler welding is not cleaned up. The reason of root crack is the cracking caused by excessive stress in root welding of girth weld.