Three　group　specimens　of　CNG　cylinder　steels,　413OX　(from　two　vendors)　and　30CrMo,　were　tested　in　dense　and　moderate　H2S　solutions.　By　using　modified　WOL　specimens　loaded　with　bolts,　we　obtained　K-ISCC　in　H2S　environment,　as　well　as　K-IC　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　K-IC　of　those　specimens　were　all　in　a　high　level,　thus　with　good　fracture　toughness　in　normal　environment.　However,　K-ISCC　in　H2S　environment　ranged　from　20%　to　40%　of　K-IC.　For　two　groups　of　specimens　made　of　CNG　steel　413OX　but　from　different　vendors,　their　K-ISCC　are　30%　different　and　their　critical　crack　sizes　are　80%　different.　The　big　difference　was　caused　by　heat　treatment　methods　in　the　manufacturing　process　based　on　our　investigation.　The　critical　crack　size　is　3.07mm　in　dense　H2S　environment　under　20Mpa　working　pressure.　In　moderate　H2S　environment,　the　critical　crack　sizes　of　specimens　of　Group　1　(413OX),　Group　2　(413OX)　and　Group　3　(30CrMo)　are　3.56　mm,　6.46　mm　and　4.55　mm　respectively.　Although　the　H2S　Concentration　of　two　corrosion　solutions　is　much　different,　there　is　no　much　difference　of　average　K-ISCC　values　in　the　two　H2S　solutions.　That　needs　further　investigation.