We have investigated the magnetostriction and the magnetization process of 3%Si-Fe single crystal with (110) parallel to the surface, when the direction of magnetization is deviated from . It was found that large contraction and expansion occurred as a result of magnetostriction in the magnetization process, even if the deviation angle was small. Complicated magnetic domain structures were observed with external field being applied. The behavior of magnetostriction corresponds to changes of magnetic domain structure. On the basis of the principle that no magnetic pole appears at domain walls, a model of domain structure is proposed. Using the model, the deformation behavior was explained well.