The Fe-Si magnetic film with an Ru underlayer was fabricated and characterized. In the film, the Fe-Si grain became small when using a very thin Ru underlayer. In addition, the surface roughness of Fe-Si film with a thin Ru underlayer became smaller than that without an Ru underlayer. Consequently, the hard axis coercivity of the Fe-Si film with a 1 nm Ru underlayer was about 1.1 Oe, which was much lower than that of the same film without an Ru underlayer. The Fe-Si film with a 1 nm Ru underlayer had a clear uniaxial magnetic anisotropy (anisotropy field Hk: 18 Oe), a hard axis relative permeability of 900, and a ferromagnetic resonance (FMR) at around 1.6 GHz. A very sharp peak of the imaginary part of permeability μh" was observed at the FMR frequency. This was due to the clear uniaxial magnetic anisotropy and small anisotropy dispersion.