Direct and continuous observation of epitaxy of fcc metals on MgO substrates in an electron microscope reveals that the shapes of nuclei at the initial stage of deposition are closely related to the characteristics of the later epitaxial growth. In order to obtain consistent results, MgO substrates are cleaved in situ at the specimen holder at the deposition temperature (300 °∼500 °C) and the metals are evaporated immediately thereafter. The nuclei are clearly found to have characteristic shapes depending on the orientations. For Au, they are square pyramids for nuclei of (100) orientation and triangular pyramids for those of (111) orientation. The application of Wulff’s law to the shapes of these nuclei yields the relative magnitude of the interfacial interaction and the surface energy. The sensitive dependence of the epitaxial characteristics on the shapes of nuclei thus indicates the important contribution of the interfacial interaction to epitaxial behavior.