Abstract: The effect of microstructure of X80 pipeline steel welded joint on its corrosion behavior in supercritical carbon dioxide (SC-CO₂) environment was studied. The microstructure of base metal (BM), fine grain heat affected zone (FGHAZ), coarse grain heat affected zone (CGHAZ), and welding metal (WM) of X80 pipeline steel welded joint after high temperature tempering were observed by metallurgical microscopy. The corrosion morphology of different zones in the welded joint corroded in gas phase environment of SC-CO₂ containing saturated water under condition of 40 ℃ and 10 MPa were observed by scanning electron microscopy (SEM), and the phase composition of the corrosion scale was analyzed by X-ray diffraction (XRD). The results show that the BM and WM consisted of different forms of ferrites. The microstructure of FGHAZ and CGHAZ were different forms of ferrites and pearlites. CO₂ corrosion could be found in all zones of the welded joint obviously in SC-CO₂ environment containing saturated water, and flowerlike iron carbonate corrosion scale islands dispersed on the surface of dense corrosion scale. Corrosion pits under the corrosion scale appeared at all zones. Localized corrosion under scale was heavy at BM, WM, CGHAZ due to Martensite-austenite (M-A) islands precipitating at grain boundaries, while it was slight at FGHAZ due to the dispersive precipitation of M-A islands in grain and grain refinement.