© 2018, The Minerals, Metals & Materials Society and ASM International. In the present work, the microstructure, micro-hardness, and chemical composition of 9Cr-1Mo weld joint after long-term service in a supercritical power plant were analyzed by using various analytical techniques. The hardness profile showed a maximum at the fusion boundary and a minimum in the region of the fine grain heat-affected zone (FGHAZ). Many cavities along the grain boundaries and in grains were also observed in the FGHAZ. Ferrite zones with lower hardness and larger grain sizes were observed in the weld metal. The formation mechanisms of the cavities and ferrite zones were discussed based on the results.