Thin layers of La, Y, Au, Pd, Pt, Ag, and Sn were evaporated at room temperature as nucleants prior to the evaporation of 2000 Å Pb. The correlations of grain size, surface morphology, and the phase diagrams of lead nucleant showed that the grain size of lead was reduced only if the nucleant formed a compound with lead and a eutectic between the compound and lead. In such a case the surface morphology was hemispherical, indicating that crystallization from the vapor took place through the liquid phase. The various sources of undercooling explaining the presence of the liquid phase are discussed. It is shown that the necessary condition for heterogeneous nucleation is the same as the condition for lamellar or rodlike eutectic growth; therefore, the occurrence of such eutectic in the phase diagram indeed indicates the possibility of heterogeneous nucleation. A vapor-liquid-heterogeneous nucleation mechanism of crystallization is suggested. Factors affecting the surface morphology of nucleated films are discussed.