Abstract: Low frequency alternating magnetic field is initiatively used by the authors to process the welded samples with internal residual stress.Experiments show that both strong pulsed magnetic treatment and low frequency alternating magnetic treatment can reduce the residual stress in materials to some extent.The way that the magnetic treatment reduces the residual stress is different essentially from that by thermal methods.Magnetic treating parameters and magnitude of stress relaxation are investigated quantitatively in this article.The microstructure change of a commercial steel during magnetic treatment is observed by transmission electron microscope and discussed from the point of view of the dislocation behavior in the processed material.The cell wall structure of dislocation in a tensively pre stressed specimen of a commercial steel 30CrMnSiA disappears and the dislocation distribution is harmonized although the dislocation density keeps almost unchanged.This harmonization of dislocation distribution is considered to be one of the main factors which lead to the residual stress reduction.