Stress evolution during initial stage of Ge nanodot formation on Si(111)-7×7 has been investigated by using simultaneous measurements of the substrate curvature and the surface morphology. In the beginning of the first bilayer growth of Ge on Si(111)-7×7, a strong compressive film stress is observed, indicating a formation of a two-dimensional wetting layer. When the layer thickness approaches the critical one for three-dimensional nanodot nucleation, a clear bend in the stress curve is observed, corresponding to a partial relaxation of the lattice planes on the surface of the nanodots. Moreover, a stress transition has been also found to exist in the very early stage of the nanodot formation, which is concurrent with the trench formation around the three-dimensional nanodots.