For application to the thermal analyses and designs of passive residual heat removal systems adopted in advanced reactors, thermal mixing characteristics in a water tank have been studied experimentally. Through the tests a fixed heat flux is supplied continuously from the two heating tubes installed horizontally or vertically in the tank until the water is saturated. According to the results, it is identified that the elapsed time to saturate the water is almost the same regardless of tube orientations. Moreover, it is demonstrated that vertical thermal stratification is formed along the tank during the heating process.