The perpendicular anisotropy caused by the internal planar stress due to the substrate constraint in Gd‐Fe and Tb‐Fe sputtered films was investigated. The internal planar stress &sgr; in these films is found to be very sensitive to preparation conditions, especially to the argon pressure during sputtering, PAr. Stress is compressive for low PArand tensile for high PAr. The contribution of the stress to the total perpendicular anisotropy varies depending on PAr. Measurement of the anisotropy change before and after the removal of the substrate reveals that the predominant part of the perpendicular anisotropy originates from the internal stress due to the substrate constraint for Tb‐Fe films with a composition of around 30 at.&percent;Tb, where the magnetostriction is extraordinarily large.