Abstract: In an ultra-supercritical boiler, the fillet weld between water wall and soot blower seal box cracked and leaked after 59 159 h service. Reason for cracking of the fillet weld was investigated by macroscopic inspection, chemical composition testing, microstructure and fracture morphology observation, hardness measurement and energy dispersive spectroscopy. The results show that the fillet weld failed by fatigue fracture. The fillet weld was subjected to relatively large axial stresses. The fillet weld leg size was smaller than the standard requirement and the weld toe transition was not smooth, leading to the increase of axial stress, and then cracks initiated. During the process of the boiler start-stop and load fluctuation, the periodical change of axial stresses caused propagation of cracks and formed fatigue cracks. The temperature of the D layer was the highest, resulting in the first cracking of the fillet weld of the D-layer soot blower seal box.