We review our first-principle total-energy electronic-structure calculations which clarify roles of energetic and kinetics in atomic reactions at the initial stage of semiconductor epitaxial growth. It is emphasized that microscopic atomic structures at the initial stages crucially affect the morphology of the resulting overlayers. The mechanisms of the (311) facet appearance, the surfactant-mediated epitaxial growth of Ge, and the substitutional adsorption of group V atoms, on the Si(100) surface are discussed.