[en] Mn behaviors in the Ge_0.96Mn_0.04 thin films grown on Si (001) substrates by molecular beam epitaxy were investigated by high resolution transmission electron microscopy, electron energy loss spectroscopy, and energy dispersive spectroscopy. Unlike the previously reported case of GeMn thin films grown on Ge, Mn has been found to be diffused toward to the surface during the thin film growth. When the Mn concentration is sufficiently high, Mn₅Ge₃ clusters may be formed. Further annealing of the high Mn concentrated thin film promotes the formation of α-Mn metallic clusters. We believe that all these extraordinary phenomena are attributed to Si as the substrate