Abstract: The leakage of fillet weld between 12Cr1MoVG ferritic steel U-tube of final superheater and 1Cr20Ni14Si2 austenitic steel hanging plate of a power plant occurred during long-term running. The reason for leakage of the fillet weld was analyzed by macroscopic observation, composition analysis, microstructure observation and hardness measurement. The results show that incompletely filled grooves were observed at the edge of the fillet weld between U-tube and hanging plate, and slag inclusions existed in the weld. At these defect positions, stress concentration was obvious. Meanwhile carbon migration occurred at fusion line on U-tube side, resulting in the increased hardness and decreased plasticity. By the stress concentration, the cracks originated at the fusion line and propagated continuously, and finally led to the cracking failure of the fillet weld.