The　effects　of　specimen　configuration,　crack　shape　and　delamination　on　fracture　toughness　have　been　investigated　experimentally　by　use　of　various　non-through　cracked　specimens　such　as　wide　and　narrow　center　surface　cracks,　double-edge　and　single-edge　corner　crack.　Comparison　with　test　results　of　through-thickness　cracks　and　mechanism　analyses　are　made　as　well.　It　was　found　that　once　crack　initiated,　severe　crack　blunting　will　occur　before　further　crack　growth　and　obvious　necking　on　the　ligament　can　be　seen.　As　the　relative　orientation　of　crack　growth　direction　with　specimen　thickness　changes　during　crack　extending,　the　coupled　effects　of　out-of-plane　constraint　and　delamination　become　quite　different　from　that　in　a　through　cracks.　Critical　stress-intensity　factors　of　initiation　for　all　the　non-through　cracks　examined　are　9.6　%-45　%　lower　than　that　of　through-cracks.　Therefore,　the　nominal　fracture　toughness　obtained　from　through-thickness　cracked　specimens　is　not　a　real　material　constancy,　instead,　it　is　a　variable　of　crack　configuration,　specimen　size　and　crack　orientation,　and　not　suitable　for　safety　assessment　of　pipelines.