Surface　heat　treatment　can　induce　micro-structural　changes　in　the　components　surface　to　form　hardened　and　transition　layer　above　the　core　zone,　which　can　be　treated　as　a　three-layer　structure.　The　case　depth　of　the　hardened　layer　is　an　important　criteria　for　evaluating　the　mechanical　properties　of　the　surface　material.　Magnetic　hysteresis　model　for　multilayer　structure　is　established　based　on　point-like　magnetic　dipole　for　quantitative　evaluation　of　case　depth.　Double　peaks　occur　in　the　predicted　differential　permeability　curve　of　three-layer　structure.　The　heights　of　the　two　peaks　and　the　height　ratio　almost　linearly　change　as　the　increasing　case　depth　to　produce　a　new　case　depth　evaluation　method.　The　measured　differential　permeability　curves,　which　are　obtained　from　high-frequency　induction　hardened　40Cr　steel　rods,　show　that　the　peak　heights　ratio　is　more　suitable　for　case　depth　evaluation　as　compared　to　coercive　force.　The　variation　coefficients　among　the　repeated　testing　results　of　the　two　peak　heights　are　lower　than　1%,　and　the　linear　regression　coefficients　are　higher　than　0.95.　The　proposed　hysteresis　model　can　act　as　theoretical　tool　for　case　depth　evaluation　using　magnetic　testing　method,　and　the　proposed　method　can　be　applied　for　plat　or　curved　components.　©　2017　Journal　of　Mechanical　Engineering.