Experiments on high pressure steam condensation inside a nearly horizontal tube were performed for application into Passive Auxiliary Feedwater System (PAFS) of Advanced Power Reactor Plus (APR+). At MIST, a Single-tube COndensation experiment facility of PAFS (SCOP) was constructed. The one-through type open loop test, where steam circulation is driven by pressure difference between the steam generator and the venting valve, was conducted in order to compensate for insufficiency of the heater power. We found out that the open loop test produced the same condensation heat transfer coefficients as those of the closed loop within 1% difference. Using the open loop, we were able to generate the higher steam flow rate by three times than that done by the heater power. Sensitivity studies were performed to validate the heat transfer performance of the PAFS in regard to the pressure effect, the diameter effect and the angle effect. The operating range of experiments are 1-7 MPa (pressure), 50.4 mm (reference tube) and 33.4 mm (comparing tube) (tube outer diameter) and 2-4 degrees (tube angle). The comparing tube gave 30% higher heat transfer coefficients over the reference tube. In our tests, the slug flow was not observed. Compared with the reference case with the inclined angle of 3 degrees, the case with the inclined angle of 2 degrees showed just a bit lower heat capacity without any transition into slug flow. (C) 2015 Elsevier Ltd. All rights reserved.