Abstract: During the process of loss of coolant accident (LOCA) in a PWR, the system pressure of primary circuit will decrease and great amount of vapor will be generated. The cooling water in the accumulator was poured into the cold leg after emergency core cooling system (ECCS) started, accompanied with temperature fluctuation and back flow phenomenon. The research was focused on thermal mixing in T-junction in ECCS and based on the experiment between subcooled water in the safe injection and saturate steam in the cold leg. The main part of this research was dedicated to obtain the effects of hydraulic jump and back flow on T-junction thermal mixing. The analyses about temperature fluctuation in the T-junction, which were based on momentum method, were attempted to find the influence of different flow patterns on the temperature distribution and a correlation to analyzing the temperature fluctuation on thermal mixing was proposed.