Fatigue　crack　growth　rates　of　4130X　steel　used　for　compressed　natural　gas　vessels　were　investigated　in　this　paper.　Considering　the　operating　conditions,　corrosion　fatigue　tests　at　a　low　frequency　of　0.0067　Hz,　in　H2S　saturated,　H2S　unsaturated　and　air　environments　were　conducted　on　modified　wedge　opening　load　specimens　by　using　a　home　made　low　cycle　fatigue　test　system.　Curve　fitting　was　applied　to　the　fatigue　test　data　of　da/dN-Delta　K　according　to　Paris　formula.　A　correlation　study　between　fracture　surface　and　stress　intensity　factor　range　was　conducted　and　K　values　for　three　stages　in　different　environments　were　characterised　quantitatively.　The　results　show　that　da/dN　in　H2S　environment　is　more　than　20　times　faster　than　in　an　air　environment.　When　the　H2S　concentration　reaches　a　certain　range,　the　increase　of　da/dN　becomes　slower　than　that　of　the　H2S　concentration.　da/dN　differs　by　2.4　times　while　the　concentration　differs　by　11　times.　The　corrosive　environment　accelerates　the　fatigue　failure.