Abstract: The microstructure and micro-hardness have been studied on the T91/G102 dissimilar steel welded joints before and after a long-time run.The results showed that the long-time run had little effect on the T91 HAZ microstructure, but had a definite effect on weld and the G102 HAZ microstructure.After the long-time run, the fine particle bainite in the welding normalized zone kept its fine microstructure and at the same time precipitated lots of fine chip-like carbides, which was proved to be M₆C carbide by TEM analyses. The lath bainite in the original matrix was transformed into tempered bainite without remaining any lath feature and precipitated lots of the block ferrite grains and chain-shape carbides instead.The results also showed that the micro-hardness on the T91 HAZ almost kept the same after its long-time run and the micro-hardness on the weld and G102 matrix dropped down.However, the micro-hardness in the heat affect zone of G102 went up unexpectedly.