The　fatigue　properties　of　2Cr12Ni2WMoV　steel　were　studied.　The　da/dN-ΔK　curves,　fatigue　thresholds　and　closure　stresses　of　fatigue　cracks　with　different　sizes　were　measured.　The　influences　of　crack　sizes　and　shapes　on　crack　growth　behaviour　and　their　closure　are　discussed.　The　results　show　that　the　fatigue　threshold　can　be　divided　into　two　parts:　intrinsic　threshold　ΔKth(i).　The　former　is　dependent　on　the　materials;　the　latter　is　mainly　controlled　by　crack　closure.　The　closure　stresses　and　thresholds　of　physical　small　cracks　are　functions　of　crack　size.　The　quantitative　relationship　between　fatigue　threshold　and　crack　length　is　established　on　the　basis　of　experiments.　©　1993.