The　magnetic　frequency　mixing　method　was　investigated　for　fatigue　damage　evaluation　of　medium　carbon　steel.　Two　types　of　magnetic　signal,　the　magnetic　flux　density　and　the　tangential　magnetic　field,　are　detected　and　analyzed.　Two　parameters,　the　magnetic　frequency　mixing　factor　and　the　power　law　coefficient,　are　proposed　for　magnetic　frequency　mixing　effect　characterization.　On-line　and　off-line　magnetic　frequency　mixing　experiments　were　performed　on　specimens　with　various　degrees　of　fatigue.　Experimental　results　show　that　the　two　parameters　changed　along　with　the　fatigue　cycles　with　monotonic　linear　regularity　and　both　parameters　can　be　used　for　fatigue　damage　evaluation　of　medium　carbon　steel.