Adsorption and diffusion of Ge atoms on a H/Si(001)-(2×1) surface is investigated by using first-principles total energy calculations and kinetic Monte Carlo simulations. The site exchange between the Ge adatom and substrate Si atoms has a significant effect on the energetics and the kinetics. We found that there are a number of adsorption geometries available for Ge atoms and that the dominant adsorption geometry temporally varies depending on the substrate temperatures. Ge atoms diffuse not on the surface but in the subsurface region. The diffusion species changes from Ge atoms to Si atoms at higher temperatures, resulting in formation of Ge-Si dimers or Si-Si dimers in the initial growth stage. These findings imply that transient states may have an important effect on the Ge growth.