A newly designed magnetic annealing apparatus was used for the treatment of Fe-3%Si steels. With the help of this device, the effect of magnetic annealing on magnetostriction was studied in a wide spectrum of external elastic stresses. It was shown that magnetostriction properties of Fe-Si steels were improved in the compressed state through magnetic annealing, while those in the unstressed state or under tensiion were found to be practically unchanged.