Abstract: A macro crack was found in T weld joint via liquid penetrating test during the failure checking process of the flange lubricator mounting base. The structure is made of Q345 steel. In order to find the cause of failure, some tests were performed, including appearance inspection of failure structure, macrocrack observation, fracture surface observation, microstructure examination, and impact fracture observation. The testing results indicate that the microstructure of welding joints have no weld defects. Obvious differences were found between the fracture morphology of the failed structure and the impact fracture morphology of weld zone. Fatigue striations at the fracture show that the failure mode is fatigue fracture. Considering the stress distribution of weld structure, it can be assumed that the cracking cause is stress concentration.