In this study, a series of epitaxial Co ultrathin films sandwiched by Au has been grown by electron beam evaporation, onto glass substrates of two different thicknesses. The Co thickness varies from I to 3 nm for each series. The dependence of the magnetic anisotropy for the two series is consistent with the variation of the mechanical film stress. We compare the magnetic properties of both series. Stress measurements are performed, by electron diffraction and optical deflection techniques, to investigate the correlation between mechanical film stress and magnetic anisotropy.