In-vessel retention through external reactor vessel cooling (IVR-ERVC) is a type of severe accident mitigation strategy. Previous works have evaluated the ERVC performance limit with critical heat flux (CHF). However, since each experimental parameter was not independently controlled, there is a limit to evaluate the effect of each parameter on CHF and their interactions. In this study, parameters were independently controlled to systematically evaluate the effects of each one on CHF. Mass flux, orientation angle, pressure, and local quality were selected as the experimental parameters, in ranges determined from the severe accident conditions in which ERVC is employed. Experiments particularly considered low orientation and pressurization conditions that have not been actively considered previously. Simultaneously, flow-visualization was performed to observe the generation and behavior of vapors according to the parameter changes to ultimately interpret the CHF tendencies. Through the parametric experiments, it was confirmed that vapor behavior directly causes CHF occurrence.