In order to achieve highly efficient spin polarized transport, first of all magnetocrystalline anisotropy energy, which determines the magnetic easy axis, must be understood. The highly precise full-potential linearized augmented plane-wave method is employed to investigate the magnetism and magnetocrystalline anisotropy energy of a ferromagnetic Heusler alloy Fe3Si on Si(001) and Si(111) substrates. The calculated magnetocrystalline anisotropy energy of bulk D0(3) Fe3Si was found to depend sensitively on a tetragonal distortion: The magnetization is along the z-axis at c/a<1 and on the xy plane at c/a>1. The out-of-plane magnetic easy axis of both Fe3Si/Si(001) and (111) was calculated to be quite stable with enhanced magnetocrystalline anisotropy energy compared with bulk value. The magnetic easy axis of Fe3Si/Si(001) and (111) is discussed in detail with single particle energy spectra. The degree of spin polarization is also presented at the interfaces between Fe3Si and Si. The calculated spin polarizations of Fe3Si/Si(111) tend to retain the spin polarization of the bulk, whereas they are reduced for the (001) interfaces. (C) 2011 Elsevier B.V. All rights reserved.