Three　groups　of　specimens　of　CNG　cylinder　steels,　4130X　and　30CrMo,　were　tested　in　dense　and　moderate　H2S　solutions.　By　using　modified　WOL　specimens　loaded　with　bolts,　we　obtained　KISCC　in　H2S　environment,　as　well　as　K1C　in　air.　The　critical　crack　sizes　of　different　materials　in　H2S　environment　were　thus　figured　out,　and　the　curves　with　working　pressures　and　equivalent　stress　to　critical　crack　size　were　plotted.　The　results　show　that　K1C　of　those　specimens　were　all　in　a　high　level,　thus　with　good　fracture　toughness　in　normal　environment.　However,　KISCC　in　H2S　environment　ranged　from　20%　to　40%　of　K1C.　For　two　groups　of　specimens　made　of　CNG　steel　4130X　but　from　different　vendors,　their　KISCC　are　30%　difference　and　their　critical　crack　sizes　are　80%　difference.