The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of the Heusler alloy Co 2 FeSi were investigated theoretically from first principles, using the fully relativistic Dirac linear MT-orbital (LMTO) band structure method. Densities of valence states, orbital and spin magnetic moments as well as polarization of the electronic states at the Fermi level are analyzed and discussed. The origin of the XMCD spectra in the Co 2 FeSi compound is examined. The calculated results are compared with available experimental data.