Nucleation and defect formation processes were observed for the heteroepitaxial growth of Ge on Si(100), (110), and (111) substrates, using an ultra-high-vacuum transmission electron microscope (TEM) combined with a conventional molecular beam epitaxial growth chamber. Successive plan-view TEM images clearly showed that the changes in the radius of Ge islands as a function of film thicknesses depend on the orientations of Si substrates. The behavior of the misfit-strain relaxation in Ge islands was found to be independent of the substrate orientations. The formation of threading dislocations was shown by discontinuity in the moire fringes between the coalesced Ge islands at 50 monolayer (ML) deposition. Twins and stacking faults, respectively, were also observed in Ge islands on (110) and (111) substrates in the early stages of 25-50 ML growth.