The effect of Sb on the formation of Ge nano islands in Si by means of molecular beam epitaxy is reported. We observe in the Ge/Si(100) system a non-monotonic dependence of the Stranski-Krastanov critical thickness of Ge islands formation on the adsorbed Sb amount. Dome- and hut-shaped Ge islands are replaced with the pyramids, when Ge is deposited on the Sb-covered Si(100) surface. The Sb-mediated conservation of the shape of Ge islands during embedding them in Si is shown. We assume that the decrease of the surface diffusion of Si and Ge ad-atoms causes these effects.