Abstract: The fracture toughness of X100 linepipe steel with different thicknesses was studied by crack tip opening displacement (CTOD) test, and the effect of the sample thickness on crack initiation value δ0.2 was discussed. The results show that crack initiation value δ0.2 of X100 linepipe steel with thicknesses of 12,10,8 mm were 0.323,0.267,0.265 mm, respectively. The thickness of the sample used during CTOD testing should be equivalent with the wall thickness of actual steel pipe.