The effect of Ge buffer layer thickness on the formation of epitaxial Mn5Ge3 films and their magnetic properties has been investigated. Epitaxial ferromagnetic Mn5Ge3 thin films were deposited on Ge/Si (1 1 1) substrates using solid phase epitaxy. The crystalline quality, chemical composition and magnetic properties of the films were investigated by x-ray diffraction, transmission electron microscopy, x-ray photoelectron spectroscopy, electron spin resonance. Increasing the thickness of the Ge layer significantly enhanced the crystallinity and magnetic homogeneity of Mn5Ge3 films. Both structural and magnetic investigations show substantial diffusion of Mn atoms through Ge buffer layer which results in the formation of Mn5Si3 and a delay in Mn5Ge3 formation until the thickness of the Ge layer reaches to 70 nm. The observed magnetic behavior for the films grown on different Ge thicknesses is interpreted in terms of changes in the magnetic phases and surface properties. These results show that Mn5Ge3 can be epitaxially grown on Si substrate with Ge buffer layer thicknesses of above 140 nm, despite ∼8% lattice mismatch between Mn5Ge3 and Si (1 1 1).