Analyses of reflection high-energy electron diffraction (RHEED) intensity changes observed during initial stages of heteroepitaxial growth of Pb on Si(111) substrates were presented. The intensity of the reflected beam is calculated by solving the one-dimensional Schrodinger equation. A simple birth-death model was used for the growth simulation in order to investigate the fundamental behaviours of reflectivity change during the growth of Pb on the Si(111) surface.