The service performance of heat resistance steels is largely determined by the precipitation kinetics. The nucleation–growth–coarsening behaviors of precipitates in G115 martensitic heat resistance steel during long-term aging at 650 °C have been systemically investigated. The microstructural characteristics, precipitate morphology and alloying element distribution were studied by scanning electron microscopy, transmission electron microscopy and scanning transmission electron microscopy. The lognormal distribution fitting combined with the multiple regression analysis was adopted to evaluate the precipitate size distributions. Laves phase has longer incubation time, and its coarsening rate is almost one order of magnitude higher in comparison with that of M23C6 carbide. Furthermore, the nucleation rate, number density, average radius, and volume fraction of two precipitates are simulated based on the classical nucleation theory and the modified Langer-Schwartz model. The precipitation behavior of Laves phase can be well explained with the Fe–W system as the interfacial energy takes 0.10 J/m2. In contrast, the simulation results of M23C6 carbide in the Fe–Cr–C system are significantly overestimated, which results from the inhibitory effect of boron on coarsening.