After TMI and Chernobyl accidents, many efforts have been made to enhance the nuclear safety with passive features. Among such passive features, the PCCS (passive containment cooling system) has been suggested by Westinghouse in the AP600 plant. The containment with PCCS is a dual containment, and consists of a stainless steel vessel and a concrete wall. In the gap between these structures, air and water can counter-currently pass and cool the steel surface. In this study, the water cooling test was performed to obtain the evaporative heat transfer coefficients in a vertical duct with one-sided heated plate. Air-steam mixture temperature and velocity, relative humidity and wall heat flux are measured to obtain the evaporative heat transfer. The measured evaporative heat transfer coefficients are compared to an analytical model which use a mass transfer coefficients. From the comparison, the predicted coefficients show good agreement with experimental data, however, some discrepancies exist when the effect of wave motion is not considered. Also, a new correlation for mass transfer coefficient is proposed.