The measurement of pressure oscillation and the observation of condensation behavior of a vapor discharge into sub-cooled liquid pool has been carried out to obtain a basic data for the evaluation of safety of the LOCA in the supercritical pressure light water cooled fast reactor (Super Fast Reactor). In the experiment, HCFC 123 is used as the test fluid. HCFC 123 is easy for handling due to its low critical pressure and temperature, and therefore, the experimental conditions can be set easily to make systematic data. The vapor at high temperature is discharged into the sub-cooled liquid pool through a submerged single pipe vertically fixed. The oscillatory condensation is observed. The condensation oscillation produces pressure oscillation in the liquid pool. The condensing interface area becomes small as the increase of the degree of sub-cooling. The pressure frequency has a period of millisecond order and the frequency and amplitude of the pressure oscillation increase with increasing the degree of sub-cooling and mass flux of the vapor, like the results of some conventional water vapor injection tests. In the present study, it is also consistently discussed the influence of the vapor temperature, mass flux, mass flow rate, back pressure of the liquid pool, pipe diameter and the degree of sub-cooling on the pressure amplitude and condensation behavior.