The　magnetic　hysteresis　behaviours　of　ferromagnetic　materials　vary　with　the　heat　treatment-induced　micro-structural　changes.　In　the　study,　the　minor　hysteresis　loop　measurement　technique　was　used　to　quantitatively　characterise　the　case　depth　in　two　types　of　medium　carbon　steels.　Firstly,　high-frequency　induction　quenching　was　applied　in　rod　samples　to　increase　the　volume　fraction　of　hard　martensite　to　the　soft　ferrite/pearlite　(or　sorbite)　in　the　sample　surface.　In　order　to　determine　the　effective　and　total　case　depth,　a　complementary　error　function　was　employed　to　fit　the　measured　hardness-depth　profiles　of　induction-hardened　samples.　The　cluster　of　minor　hysteresis　loops　together　with　the　tangential　magnetic　field　(TMF)　were　recorded　from　all　the　samples　and　the　comparative　study　was　conducted　among　three　kinds　of　magnetic　parameters,　which　were　sensitive　to　the　variation　of　case　depth.　Compared　to　the　parameters　extracted　from　an　individual　minor　loop　and　the　distortion　factor　of　the　TMF,　the　magnitude　of　three-order　harmonic　of　TMF　was　more　suitable　to　indicate　the　variation　in　case　depth.　Two　new　minor-loop　coefficients　were　introduced　by　combining　two　magnetic　parameters　with　cumulative　statistics　of　the　cluster　of　minor-loops.　The　experimental　results　showed　that　the　two　coefficients　monotonically　linearly　varied　with　the　case　depth　within　the　carefully　selected　magnetisation　region.