Abstract: Microstructure and impact toughness of F12 steel welded joint after service at high temperature for 165 000 h was studied by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, X-ray scattering technique and impact testing. Results show that after service at high temperature for 165 000 h, martensite in the welded joint of F12 steel decomposed, and precipitation phases in the welded joint mainly were M23C6 carbide, and they precipitated at the grain boundaries, and then gathered and grew with isolated granule or chain-like distribution. The welded joint exhibited obvious aging brittleness, this attribured to gathering and growing of M23C6 phase. The base metal in the joint was intergranular fracture morphology, and the weld metal was cleavage fracture morphology.