Development of magnetism based non-destructive test (NDT) technologiesrequires quantitative relations between deformation, stress,damage or fracture of structures and their induced magnetic field which canbe measured as indicators to assess integrity of the structures. Based onlinearized magnetoelastic theory, the perturbed magnetic fieldsinduced by mechanical stress and deformation in steel or otherferromagnetic structures working in the environment of the Earth magneticfield, are investigated theoretically in this paper.Governing equations and boundary conditions to determine the perturbedfields are derived. The effect of mechanicaldeformation on the magnetic fields is taken into account by couplingstructural displacement into continity conditions of the perturbed magneticfield on the boundary of the structure. When the applied magnetic field isweak, such as the Earth's magnetic field, the effect of magnetic fields onstructural deformation can be neglected. This greatly simplifies thecoupling equations of magnetomechanical interactions. It shows that thenormal projection of displacement gradient on the structure boundary plays adominating role in the perturbed magnetic fields.As an example, the perturbed field of a half-plane magnetized structurecaused by a point force is calculated by the Fourier transform method. Thecalculated magnetic intensity component normal to the boundary of thestructure assumes a symmetric distribution about the point where theforces is acted andreaches its maximum at that point while the component tangentto the boundary is asymmetric and inverses its direction sharply at thatpoint. The magnetic flux density of the perturbed fields isproportional to the magnitude of the applied force. These features provide apossible way for NDT technologies.