Abstract: Using a commercial CFD code STAR-CCM+ and adopting rational meshing technology and physical models, an approximate 1∶1 scale numerical simulation model for Monju Prototype Fast Reactor upper plenum was established, a numerical investigation of thermal-hydraulic phenomenon under 40% rated power output condition was carried out, and the relatively complete process of thermal stratification within upper plenum was studied. The results show that the steady thermal stratification is set up at 2 min after reactor shutdown and gets across the top of the up-flow barrel during 10-21 min, the flow pattern is unstable during 10-140 min when thermal stratification locates near the top of the up-flow barrel, and stable flow pattern close to that before reactor shutdown is resumed at 140 min after reactor shutdown. By comparing with test result, it is shown that a good agreement is achieved at an early stage of the transient (0-4 min), which can be concluded that the present numerical simulation is applicable to investigate the thermal stratification phenomena. It is also demonstrated that the elevation of the interface of the thermal stratification is overestimated after 4 min from the reactor trip, which is largely due to the difference of simulation boundaries and structure with the reality.