Tensile-compressive　fatigue　tests　were　performed　on　40Cr　medium-carbon　alloy　steel　by　the　controlled　stress　fatigue　method.　With　the　aid　of　an　in　situ　fatigue　damage　test　apparatus,　the　fatigue　samples　were　cycled　to　different　damage　degrees,　unloaded　from　the　fatigue　test,　tempered　at　high　temperatures,　and　then　the　fatigue　test　was　resumed　on　these　samples　under　the　same　conditions.　This　makes　it　possible　to　study　the　influence　of　the　intermediate　high　temperature　tempering　treatment　on　the　fatigue　life　of　the　steel.　Experimental　results　show　an　obvious　enhancement　of　the　fatigue　life　for　the　intermediately　tempered　samples,　with　20%　increase　for　the　samples　tempered　at　0.3　Nf,　40%　increase　for　those　tempered　at　0.4　Nf,　and　77%　increase　for　those　tempered　at　0.5　Nf.　The　main　reason　for　the　fatigue　life　enhancement　is　correlated　with　the　enlarged　dimples　and　more　fatigue　sources　on　the　fracture　surface.