Abstract: Reactor pressure vessel (RPV) is very significant in terms of nuclear plant safety. In this paper, using molecular dynamics and Monte Carlo methods, the mechanism of solute precipitation in model alloy FeCu_0.5Ni_0.8Mn_1.5 after thermal ageing was studied. The results show that after thermal ageing, Cu enriched precipitate is formed, meanwhile solute elements, such as Ni and Mn, are aggregated on grain boundaries (GBs). The segregation of these elements can enhance the strength of GBs.