The nucleation and growth of Si onto Si(100) has been studied during deposition in ultrahigh vacuum (UHV) by spot profile analysis of low‐energy electron diffraction (SPA‐LEED). The growth mode and surface atom arrangement have been determined not only by the intensity oscillation of the 00 beam during deposition but also after interruption and cooling by a detailed measurement of the energy dependence of the spot profile. We present two different new evaluation procedures to determine out of the structure factor G(0,K⊥) the layer distributions, i.e., the roughness of the surface, and deposited layers. During deposition a rough surface is always observed with a root mean square value (rms) of Δ=0.75 Å for the fist layer up to Δ=0.9 Å for the 50th layer. In contrast to Si(111), which starts growing islands with double step height (d=6.27 Å), for Si(100) it is found the minimum possible step height of a0/4=1.36 Å for neighboring terraces, i.e., a monoatomic growth mode. The resulting terrace width distribution of the islands on the surface is a geometric distribution with an average terrace length of 16 atoms and an island density of about 6×1011 per cm2 at 415 °C substrate temperature.