During a loss-of-coolant accident (LOCA) in the pressurized water reactors (PWR), the emergency core cooling system (ECCS) is actuated and cold water is injected into cold legs. Insufficient mixing between injected cold water and hot primary coolant results in the thermal stratification phenomenon, which is the key concern to assess the pressurized thermal shock (PTS) in terms of aging and licence renewal of the nuclear power plant. In this study, unsteady simulation of single phase thermal stratification in both cold legs and downcomer by emergency core cooling system injection were conducted with commercial CFD software ANSYS CFX R.17. Predicted results were compared to data measured from ROSA/LSTF (Large Scale Test Facility).