Ge0.975Sn0.025 alloy films have been grown on Si(001) substrates by molecular beam epitaxy with high-quality Ge films as buffer layers.The alloys have high crystalline quality without Sn surface segregation, determined by double crystal X-ray diffraction and Rutherford backscattering spectra measurement. In addition, the Ge0.975Sn0.025 alloy has rather good thermal stability at 500 ℃, which makes it possible to be used in Si-based optoelectronic devices.