The initial stage of Ge heteroepitaxy on a Si(100)‐2×1 surface has been investigated by low‐energy electron diffraction (LEED) and Auger‐electron spectroscopy (AES). The growth mode of the Ge films was studied by measuring the decrease in the Si(LVV) AES line at 92 eV with an increase in the Ge overlayer thickness. The Ge films deposited at room temperature exhibit layer‐by‐layer growth up to at least six monolayers. When the substrate is heated up to 350 °C, the growth mode is characterized by the Stranski–Krastanov type; i.e., the first three monolayers of growth is followed by island formation. Although these characteristics of the growth mechanism are similar to the case of Ge on Si(111)‐7×7 surfaces, annealing behavior of the Ge films suggests that the bond strength between Ge and Si is stronger on Si(100) than on Si(111) surfaces. In contrast to the case of Ge on Si(111) surfaces, where the original 7×7 superstructure of the Si surface is replaced by a new 5×5 pattern at about two‐monolayer coverage of Ge, the original 2×1 LEED pattern is not strongly disturbed up to about 1–2 monolayers of Ge. In addition to the detailed study on the initial stage of heteroepitaxial growth, we observed that thick Ge films deposited onto Si(100) surfaces held at 350 and 470 °C display a sharp 2×1 LEED pattern and demonstrate a single‐crystal growth of a Ge(100) face on the Si(100) surface. This is further supported by a measurement of the x‐ray diffraction pattern of the Ge films.