The previous investigations were mainly conducted under the condition of low pressure, however, the steam-water specific volume and the interphase evaporation rate in high pressure are much different from those in low pressure. Therefore, the new experimental and theoretical investigation are performed in Xi'an Jiaotong University. The investigation results could be directly applied to the analysis of loss-of-coolant accident for pressurized water reactor.The system transition characteristics of cold leg and hot leg break loss-of-coolant tests are described for convective circulation test loop. Two types of loss-of-coolant accident are identified for "hot leg" break, while three types for "cold leg" break and the effect parameters are studied. Critical flow rate is dependent not only on the upstream fluid properties but also on the break geometries. Tests indicate that the mass flow rate with convergent-divergent nozzle reaches the maximum value among the different break sections at the same inlet fluid condition because the fluid separation does not occur. A wall surface cavity nucleation model is developed for prediction of the critical mass flow rate with water flowing in convergent-divergent nozzles.