The condensation heat transfer characteristic curves for binary mixture of water and ethanol vapors in the range of relatively small concentration of ethanol vapor, i.e., mass fraction of 0, 0.0002∼0.20, were measured by using the leak-tight experimental apparatus. The variations of the several characteristic points representing the characteristic curves were discussed. It was also shown that the excellent heat transfer characteristics were achieved in the low ethanol concentration because the surface subcooling region of the diffusion resistance is narrow and the variation of the surface tension is large in this domain. Especially the maximum heat flux was revealed at about the mass fraction of 0.06 and the maximum heat transfer coefficient appeared at about the mass fraction of 0.02 under very low concentration of non -condensable gas (about 10 ppm).