Results on the correlation of structure and magnetic properties in ARMCO steels after mechanical treatment are reported in this paper. Samples have undergone plastic deformation by means of cold rolling and tensile stress in their as-bought state and after stress-relief process. Minor magnetic hysteresis measurements in 1 Hz have been performed in the as-bought, annealed and plastically deformed samples. X-ray diffraction, optical metallography and transmission electron microscopy have been employed to determine the microstructure of the samples. A clear decreasing monotonic dependence of the maximum differential permeability on applied stress and dislocation density has been observed.