A corrosion failure of materials, often exposed to the corrosive environment in the district heating facility, has a critical influence on structural integrity. Thus, it is very important to investigate the cause of the failure for the establishment of effective prevention methodology. In this study, the failure analysis was performed in the case that a water wall tube for a boiler in district heating was fractured along the axial direction of the tube that is SA-210 Grade A1, which is frequently used for the boiler. The feed water temperature was about 220°C and the operating pressure was about 10 MPa. For the visual inspection of the failed part, the wall thinning and failure were observed in the location where the heat was directly reached. Sodium and iron oxides were only found in the inner surface of the tube, as confirmed by x-ray diffraction, and it indicates that hydrogen is generated by the mechanism of the caustic corrosion, resulting in hydrogen embrittlement near the inner surface. The microstructural changes along the direction of radial and hoop were intensively examined. A number of microvoids were formed along the grain boundary, leading to some microcracks in the location inside the tube. As a result, it is concluded that an unstable magnetite film at elevated temperature under tensile residual stresses developed along the hoop direction in the interior of the tube undergoes the caustic corrosion, and microvoids or microcracks along the grain boundary formed due to hydrogen embrittlement cause a final fracture of the tube.