Grade 91 steel has high creep strength and has been used as the material of piping in 600°C USC power plants in China. The Grade 91 materials used in actual power plants are useful in estimating the changes of material properties caused by long-term aging and damage at low stress conditions. An understanding of the long-term microstructural evolution under actually used conditions is a key for the improvement of these heat resistant steels. In this article, microstructural evolution of Grade 91 steel under different service conditions in Chinese power plants was studied using optical microscope (OM), transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) techniques. The results show, M23C6, MX (V-rich particles; Nb-rich particles), and Laves phases were found to precipitate. A quantitative characterisation of microstructure evolution was evaluated during long-term exposure, focusing on the size of precipitates (M23C6 carbides, Laves phase, MX phase) for the Grade 91 steel after long-term service.