The influence of stresses on the magnetic properties of non-oriented electrical steels was studied. The dependence of iron loss on compressive stress was affected by grain size. The magnetic polarization J at high magnetic fields such as 5000A/m and 10000A/m increased by compressive stresses and decreased by tensile stresses in samples with low Si contents. Using Kerr-effect domain observation, it was found that the reduction in J caused by tensile stresses was attributable to residual striped domains. Magnetostriction measurements at high magnetic fields indicated that the increase in J under compressive stresses originated from the Villari-effect due to negative magnetostriction in low Si materials.