The　mixing-frequency　method　is　an　effective　technique　to　measure　the　magnetic　nonlinearity　of　medium　carbon　steel.　In　this　paper,　the　magnetic　mixing-frequency　method　was　investigated　to　evaluate　the　hardness　of　medium　carbon　steels.　The　magnetic　nonlinear　factors　related　to　the　bispectral　amplitudes　at　sum　frequency,　different　frequencies,　and　fundamental　frequencies　in　the　measured　signal　are　proposed　for　hardness　evaluation.　Nonlinear　magnetic　mixing-frequency　measurements　were　conducted　on　samples　of　medium　carbon　steel　tempered　at　different　temperatures.　The　experimental　results　showed　that　the　magnetic　mixing-frequency　technique　is　an　effective　way　to　measure　the　hardness　of　medium　carbon　steels　and　the　magnetic　nonlinearity　is　more　sensitive　to　the　change　of　hardness　than　magnetic　Barkhausen　noise　measurements.　The　proposed　method　has　a　potential　for　use　in　the　mechanical　properties　measurement　of　ferromagnetic　steel.