A theoretical model is presented to quantitatively account for the magnetostrictive behavior of grain oriented 3% SiFe laminations as a function of induction B and of applied tensile stress. Static magnetostriction measurements on high permeability longitudinal laminations show that the elongations Δ1/1 are negative for all B values, except close to saturation, reaching a deep minimum peak forB cong 1.75T. The Δ1/1 and in particular the peak absolute values are found to strongly decrease upon application of longitudinal stresses (up to 12 kg/mm2). In the model the behavior is considered of 90° spike domains, which characterize the surface of grains whose easy axis makes an angle ϑ with the lamination plane, to reduce the demagnetizing field energy. By minimizing the grain magnetic energies, the 90° domain volume is expressed as a function of the magnetization state. In particular it is found that spikes tend to disappear above a critical field which depends on the grain angle ϑ. The contributions to the magnetostrictive elongation deriving from these 90° domain volumes are then averaged over the actual distribution of grain orientations for the investigated g.o. material, and the theoretical curve is found to be in good agreement with the experimental one. The effect of stress is also considered, and it is shown that under tension the total 90° domain volume is strongly reduced, thus accounting, again in good agreement with the experiments, for the decrease of the negative magnetostriction peak amplitude.