Corrosion failure of materials often exposed to the corrosive environment in district heating facility has a serious effect on operation and longevity of the facility. Therefore, it is very important to establish an effective prevention methodology by analyzing the cause of the failure. In this study, failure analysis was performed in the case that water wall tube for boiler in district heating system was fractured along the longitudinal direction of the tube. The material of the tube was SA-210 Grade A1, which is mainly used for the boiler. The feed water temperature was 220°C and the operating pressure was 10 MPa. For the analysis, visual inspection of the failed part was performed first. It was observed that the failure with a significant wall thinning was located in region subjected to a flame, and the surface was examined by XRD/SEM/EDS. Sodium and iron oxides were measured on the inner surface of the tube, and it indicates that hydrogen was generated by caustic corrosion. In addition, the microstructural changes in the radial direction and the hoop direction were analyzed. It was confirmed that many cavities or micro-voids were distributed around the cracks, especially along the grain boundaries only inside the tube. Furthermore, high temperature creep promoted the evolution of the cavities only inside the tube where there were tensile residual stresses. Consequently, a combination of hydrogen embrittlement and thermal creep initiated the cracks at the grain boundaries inside the tube, and the cracks propagated at the grain boundaries along the longitudinal direction exhibiting an intergranular feature on the inside. Then, it gradually propagated toward the outside, finally exhibiting a transgranular fracture mode on the outside.