The SiGe dewetting from Si(100) is found to take place during Ge deposition at 750–950 °C. The dewetting provides the formation of compact islands with lateral sizes from about 0.1 to 3 μm, large contact angles (θ) and aspect ratios up to 0.8 depending on the deposited Ge amount (d) and the Ge deposition rate. We derived an expression for the total energy of the structures with islands in the sphere segment shape that is typical at dewetting. Its analysis revealed that the use of such island shapes is valid if the strain energy at the island/substrate interface is less than the surface energy of the bare substrate areas. The energy as a function of θ for a given d allowed obtaining the dependence of θ on d, when the dependence on d was introduced via the strain energy, using the experimental data for the SiGe island composition measured for different d. The minimum is shallow for the relatively large d that explains the large scattering of θ observed in the experiment. The comparison of the obtained dependence of θ on d with experimental results allowed estimating the effective thickness of the strained layer around the island/substrate interface.