The　effects　of　three-dimensional　crack　configurations　and　delaminations　on　fracture　mechanism　and　fracture　toughness　J(c)　of　pipeline　steel　were　investigated　experimentally　by　use　of　tensile　specimens　having　surface　cracks　of　different　depth　to　length　ratio.　Comparison　with　test　results　of　through-thickness　cracks　and　mechanism　analyses　are　made　as　well.　When　3D　stress　constraint　is　larger　than　the　strength　in　the　thickness　direction　the　delamination　forms.　As　no　delamination　occurs　in　the　interior　of　a　surface　crack,　the　constraint　is　higher　and　the　fracture　toughness　is　lower　than　that　of　the　through-thickness　cracks.　Therefore,　the　nominal　fracture　toughness　obtained　from　through-thickness　cracked　specimens　is　not　a　real　material　constancy,　and　not　suitable　for　safety　assessment　of　pipelines.