Abstract: Optical microscopy, scanning electron microscopy and transmission electron microscopy were used to study microstructure and mechanical properties of Super304H superheater tube after serving in ultra-supercritical unit of a power plant for 10 000 h. The results show that nano-precipitates despersively distributed in unserved superheater tube, and no precipitates gathered at grain boundaries, so the tube had good high-temperature strength and good plasticity. The grains in served superheater tube were coarsening, and amounts of and continuous M23C6 precipitates and coarse Nb(C, N) precipitates precipitated at grains boundary, so its ductility decreased significantly.