S31042 steels with 25%Cr (mass fraction) and 20%Ni have been served as super-heaters and re-heaters in ultra-super critical (USC) plants, owing to their outstanding corrosion resistance and creep rupture strength. And the reliability of joints at high temperature has attracted much attention since the S31042 steels have been joined successfully by linear friction welding. In this work, the microstructures and mechanical properties of linear friction welded S31042 steel joint subjected to ageing treatment were investigated by using OM, SEM, TEM and mechanical test at 700 ℃. The recrystallized grains and nanoscale NbCrN particles have been stable during the high-temperature ageing, and the joint exhibited excellent performance due to the grain refinement strengthening and precipitation strengthening. The average size of M₂₃C₆ phase in weld zone, thermo-mechanically affected zone and heat affected zone increased with the ageing time. After ageing treatment at 700 ℃ for 500 h, σ phase precipitated at boundary junctions in thermo-mechanically affected zone. The average size of σ phase increased with the ageing time, as well as the volume fraction of the σ-phase. With the formation of σ phase, the fracture site of joints shifted from the parent material to the areas adjacent to the weld zone, and the high-temperature mechanical properties of joints were sharply decreased.