Residual　stress　caused　by　magnetic　field　treatment　is　studied　through　magnetostriction　investigation.　The　external　magnetic　field　is　carried　out　for　both　stress　relief　and　magnetostriction　at　the　same　time.　Samples　of　steel　45　are　produced　by　tungsten　inert　gas　welding　and　different　heat　treatment　temperature　and　time　are　used　to　modify　stress　levels.　The　magnetostriction　in　the　magnetic　treatment　process　is　recorded　and　residual　stresses　before　and　after　the　action　of　magnetic　are　compared.　Experiment　results　shows　that　magnetostriction　behaviors　with　different　stress　levels　are　different　obviously.　However,　a　good　coherence　exists　between　the　relative　change　of　magnetostriction　and　that　of　residual　stress.　Thus　residual　stress　relief　during　the　magnetic　treatment　can　be　evaluated　by　magnetostriction　investigation　conveniently.　To　explain　the　relationship　between　magnetostriction　and　stress　level,　energy　differential　equations　are　constructed　and　energy　boundary　conditions　are　modeled.　The　results　deducted　from　the　established　equations　show　that　the　product　of　magnetostriction　variation　caused　by　pulsed　magnetic　field　and　residual　stress　in　macro　scope　is　constant,　which　agrees　well　with　experiment　results.　©　2010　Journal　of　Machanical　Engineering.