Abstract: Mechanical properties of ten girth welds with defects in X80 steel gas transmission pipelines were measured and causes of two types of cracks were analyzed. The results show that the unqualified rate of tensile strength of 10 girth welds was 10%. The unqualified rate of impact energy of the weld center was 21.7%, while the impact energy of the heat-affected zone was all qualified. The Vickers hardness of the girth weld, base metal and heat-affected zone all met the requirements, but softening occurred in the heat-affected zones. The main defects in the girth welds were incomplete fusion and cracks; the ratio of the two was 75%. The cracks included cold cracks and solidification cracks. The cold crack originated from the incomplete fusion at the weld root, and propagated under restraint stresses, leading to cracking. The reason for the solidification crack was that sulfur segregated on grain boundaries in center of the backing weld and formed low melting point eutectic phases, and then the grain boundaries cracked under tensile stresses caused by solidification shrinkage.