The accurate understanding and prediction of neutron irradiation embrittlement of reactor pressure vessel (RPV) steels is a very important issue to ensure the safety and continued operation of nuclear power plant. High-density nano-scale Cu-rich precipitates, which are typically alloyed with Si, Ni or Mn, have been considered as the main contributor to the hardening embrittlement of RPV steels. In our work, four series of RPV model alloys, Fe-Cu, Fe-Cu-Si, Fe-Cu-Ni and Fe-Cu-Ni-Mn, are thermally aged at 450˚C for up to 380 hours. The effects of Si, Ni and Mn on hardening are discussed. Furthermore, Atom Probe Tomography (APT) was used to study Cu-rich precipitates.