This Paper describes expedmenm|and numerical investrga. tlons of sub-cooled boiling, condensadon, and void flashing in the HRTL-5 test loop, which simulates the primary loop of a 5 MW nuclear heating reactor. A drift-flow model of two-phase with four govenffng equations was used, in which sub-cooled boiling, eondensalion nnd void flashing hove been taken into account. Based on the mathemadcal model, a program has been developed for analyzing the namral cir-culation system. As parameters, inlet sub-cooling, system pres-sure, and heat flux are varied. For comparison, some simpli-fied modeis, which are designed to reveal the importance of sub-cooled boding, condensation, flashlog in the HRTL-5 test loop. are adopted in the program. The results show: (1) sub-cooled boiling. condensatian, and void flashing may have great influence on the distributlon oF the void fracdon and morP intense at low system pressure; (2) the calculaiion of them is correlatire and interactive other than independent; (3) for a system with short heated sealon. long riser, and low pressure, it is possible to reach "boiling out of the core", where is almosl no void in the heated seelion bul much in the riser.