Abstract: Cracking occurred on the branch pipe side of an equal diameter seamless tee during the process of pressure testing and putting into production. The cracking reason was analyzed by morphology and microstructure observation, nondestructive testing, and chemical composition and mechanical property tests. The results show that the cracking location of the failed tee was on the girth weld of the branch pipe, and the girth weld was produced in the manufacturing stage of the tee, which did not meet the production quality requirements of the equal diameter seamless tee. The main reason for cracking was that there were serious incomplete penetration defects in the girth welds. With increasing test pressure, cracks were initiated at the incomplete penetration defects and expanded until penetrating the remaining wall thickness.