To improve condensation heat transfer performance in a variety of systems, reduced channel sizes are used. However, few studies have been performed on complete heat exchangers. Hence, condensation heat transfer coefficients were studied experimentally in two ultracompact heat exchangers with a hydraulic diameter of 133 μm using steam as the working fluid. Effects of mass flux, average vapor quality, saturation pressure, and heat exchanger size were examined. The condensation heat transfer coefficients showed strong influence of mass flux and quality. However, the effects of saturation pressure and heat exchange size were not significant. Three conventional and three mini/microscale correlations were compared with the experimental data. The conventional and mini/microscale correlations developed for annular flow overpredict the data significantly. The Soliman correlation developed for mist flow showed the best agreement with the data.