This study investigates the microstructure evolution of Type 316H stainless steel, focusing on the identification of major precipitates using advanced characterization techniques. The precipitation sequence at service temperatures of 650°C is identified as M 23 C 6 , followed by Laves phase, grain boundary (GB) sigma phase, and inter-granular sigma phase. At 750°C, the sequence progresses from M 23 C 6 to Laves phase, GB sigma phase, chi phase, and intra-granular sigma phase, with the chi phase forming intra- and inter-granularly after 5,000 hours of aging. During the formation of the sigma and chi phases, carbides and Laves phases dissolve. A Monte Carlo model has been developed to predict detailed microstructure evolution during long-term aging, calibrated using quantitative precipitate evolution measurements of Type 316H. After validation, the model aligns well with experimental data, offering a method to predict the microstructure of Type 316H and potentially other austenitic stainless steels over the lifespan of power plants.