A welded joint of Mod.9Cr-1Mo steel was creep tested to elucidate the mechanism of Type IV failure in weldment. Its microstructure was characterized, before and after the creep test. Cavities are nucleated at coarse M 23C6 carbides on grain boundaries, and their growth is assisted by creep deformation concentrated in the area of fine grain size and coarse precipitates. Strain heterogeneity among grains can assist the cavity formation. Growth rate of M23C6 precipitates is extraordinarily higher in intercritical (IC) HAZ as compared to other locations, resulting in large M23C6 size in ICHAZ. The larger size of M23C6 precipitates determines the initiation of Type IV failure in ICHAZ. Hardness and image quality (IQ) index change drastically in ICHAZ before post-weld heat treatment (PWHT) due to the change in volume fractions of martensite grains and fully annealed ferrite grains. The drastic change in hardness disappears after PWHT, but that in IQ index remains. This finding suggests the IQ index to be a useful measure for identifying ICHAZ.