Abstract: The mechanism of the formation of initial condensate droplets for dropwise condensation is still not clear. Magnesium was used as condensation surface in this study since it can react with hot water (condensate) and thus make the chemical composition of the surface changed. In the experiments, the initial condensation of steam on the magnesium surface was achieved in the designed apparatus by controlling subcooling and condensation time. Then, the test surface was scanned with an electron probe microanalyzer (EPMA) for the variation of the chemical composition of the surface before and after the initial condensation,which can be used to deduce the state of the initially formed condensate, whether in nuclei or in thin film. The results showed that the oxygen contents on the test surface increased with subcooling and condensation time obviously after initial condensation. And the oxygen on the test surface was not distributed uniformly. It indicated that the initial condensate was formed in nuclei on the solid surface,but not in thin film. Therefore, the mechanism of the formation initial condensate droplets for dropwise condensation accords with the hypothesis of surface nucleation sites.