The nanostructures and magnetic properties of Ge1-xMnx thin films grown on Si substrates by molecular beam epitaxy, with different nominal Mn concentrations (1-4%) and different growth temperatures, have been systematically investigated by transmission electron microscopy and superconducting quantum interference device. It was discovered that when Ge1-xMnx thin films were grown at 70 degrees C, with increase in Mn concentration, Mn-rich tadpole shaped clusters started to nucleate at 1% Mn and become dominate in the entire film at 4% Mn. While for the thin films grown at 150 degrees C, tadpoles was firstly seen in the film with 1% Mn and subsequently Mn-rich secondary precipitates became dominant. The magnetic properties show specific features, which are mainly related to the nature and amount of Mn-rich clusters/precipitates within these thin films. (C) 2010 Elsevier B.V. All rights reserved.