We have investigated the nucleation and evolution of germanium(Ge) nanodot (ND)s taking place while depositing Ge onto the silicon (Si) (1 1 1) surfaces with ultra-thin Si oxide films by using ultra-high vacuum in situ high-resolution transmission electron microscopy in the profile-imaging geometry. Various types of growth phenomena such as nucleation, growth and coalescence of Ge NDs have successfully been observed. The results show that the growth phenomena of the Ge NDs are dramatically rapid after their size reaches the size of the critical nucleus. The critical nucleus size estimated from a model using the cohesive energy of the Ge NDs has been consistent with observed one. (C) 2008 Elsevier B.V. All rights reserved.