Development　of　magnetism　based　nondestructive　testing　technology　(NDT)　and　the　micro-electronic　mechanical　system　(MEMS)　require　accurate　computation　of　perturbed　magnetic　fields　caused　by　mechanical　stress.　In　this　paper,　based　on　the　linearized　magnetoelastic　theory,　governing　equations　and　boundary　conditions　to　determine　the　perturbed　magnetic　fields　were　derived　for　the　case　of　weak　external　magnetic　fields　such　as　the　Earth　magnetic　field.　Under　those　weak　magnetic　fields,　effect　of　the　magnetic　fields　on　mechanical　deformation　was　reasonably　neglected　in　order　to　decouple　the　interaction　between　deformation　and　magnetic　field.　The　effect　of　deformation　on　the　perturbed　magnetic　field　is　found　out　by　introducing　the　displacement　gradient　into　the　boundary　conditions　the　perturbed　field　should　hold.　As　an　example,　analytic　solution　of　the　perturbed　magnetic　field　of　an　infinite　plate　with　a　round　hole　under　tensile　stress　has　been　obtained　by　the　procedure　presented.　The　results　show　that　the　perturbed　magnetic　field　induced　by　stress　is　3　orders　less　in　magnitude　of　intensity　than　that　of　magnetic　fields　without　stress,　and　it　has　prominent　local　features　such　as　having　more　peaks　and　decaying　more　rapidly　in　the　radial　direction　of　the　hole　than　the　case　of　stress-free.