A computational study of a thermal striping in the upper plenum of PGSFR(Prototype Generation-IV Sodium-cooled Fast Reactor) being developed at the KAERI(Korea Atomic Energy Research Institute) is presented. The LES(Large Eddy Simulation) approach is employed for the simulation of thermal striping in the upper plenum of the PGSFR. The LES is performed using the WALE (Wall-Adapting Local Eddy-viscosity) model. More than 4.0 million unstructured numerical grids are generated in upper plenum region of the PGSFR using the CFX-Mesh commercial code. From these results, the time-averaged velocity components and temperature field in the complicated upper plenum of the PGSFR are calculated. The time history of temperature fluctuation at the dozen locations of solid walls of UIS(Upper Internal Structure), Control Rod Shroud Tube and IHX(Intermediate Heat eXchanger) are compared with both results and are additionally stored. It has been confirmed that the most vulnerable regions to thermal striping are the first plate of UIS and the flow hole at the side of UIS. From the temporal variation of temperature at the solid walls, it was possible to find the locations where the thermal stress is large and need to assess whether the solid structures can endure the thermal stress during the reactor life time.