The microstructure, chemical composition and oxidation of the ex-service steam tube containing T23/TP316 H dissimilar metal weld were investigated to analyze damage after service. The results show that after service for 100 000 hours at 540 ℃/17 MPa,although the TP316 H steel side is oxidized,no potential risk that may cause early failure is found;however,the T23 steel side is oxidized along the weld/heat affected zone interface and cracks along the interface is observed. The coupling interaction between interfacial oxidation damage of weld metal, the differences in thermal expansion coefficient and creep strength on both sides of the interface,and the hardness gradient near the interface is likely to induce interfacial failure and increase the early failure risk of the welded joint. In addition,some common detrimental factors such as creep and soft zone induced by carbon migration are not found at the T23 steel side,thus interfacial oxidation damage is the main damage mode of joint studied in this paper.