Development　of　magnetism　based　non-destructive　test　(NDT)　technologies　requires　quantitative　relations　between　deformation,　stress,　damage　or　fracture　of　structures　and　their　induced　magnetic　field　which　can　be　measured　as　indicators　to　assess　integrity　of　the　structures.　Based　on　linearized　magnetoelastic　theory,　the　perturbed　magnetic　fields　induced　by　mechanical　stress　and　deformation　in　steel　or　other　ferromagnetic　structures　working　in　the　environment　of　the　Earth　magnetic　field,　are　investigated　theoretically　in　this　paper.　Governing　equations　and　boundary　conditions　to　determine　the　perturbed　fields　are　derived.　The　effect　of　mechanical　deformation　on　the　magnetic　fields　is　taken　into　account　by　coupling　structural　displacement　into　continity　conditions　of　the　perturbed　magnetic　field　on　the　boundary　of　the　structure.　When　the　applied　magnetic　field　is　weak,　such　as　the　Earth＇s　magnetic　field,　the　effect　of　magnetic　fields　on　structural　deformation　can　be　neglected.　This　greatly　simplifies　the　coupling　equations　of　magnetomechanical　interactions.　It　shows　that　the　normal　projection　of　displacement　gradient　on　the　structure　boundary　plays　a　dominating　role　in　the　perturbed　magnetic　fields.　As　an　example,　the　perturbed　field　of　a　half-plane　magnetized　structure　caused　by　a　point　force　is　calculated　by　the　Fourier　transform　method.　The　calculated　magnetic　intensity　component　normal　to　the　boundary　of　the　structure　assumes　a　symmetric　distribution　about　the　point　where　the　forces　is　acted　and　reaches　its　maximum　at　that　point　while　the　component　tangent　to　the　boundary　is　asymmetric　and　inverses　its　direction　sharply　at　that　point.　The　magnetic　flux　density　of　the　perturbed　fields　is　proportional　to　the　magnitude　of　the　applied　force.　These　features　provide　a　possible　way　for　NDT　technologies.