In recent years there has been theoretical and experimental interest in the properties of Fe thin layers grown on metallic substrates. We present here a theoretical study based on ab initio calculations of the structure and magnetic characteristics for a very thin Fe film grown on an As-terminated MnAs substrate, which is metallic. Particular emphasis is put into the relaxation and reconstruction of the interface and on the characterization of the magnetic coupling with the substrate. The absence of Fe interdiffusion and the presence of an intermediate As layer ensure the partial magnetic decoupling between epilayer and substrate.