Due to the extensive applications of large diameter/thickness and higher pressure gas transmission pipelines, and there will be an increasing need for reliable pipeline design and failure assessment that will preclude catastrophic accident. Specifically, the actual fracture toughness needs to be determined accurately. The present work innovatively correlate the material’s fracture toughness with the crack-tip geometric constraint effect by using the crack-tip plastic zone. The significant “thickness effect” impact on pipeline steel’s fracture toughness is elucidated by the proposed out-of-plane constraint factor 1α out . The critical loads ( F C i ) of three groups of thin thickness specimens at fracture are recorded by the three-point bending tests performed on the single-edge notched (SENB) specimens, corresponding fracture toughness are calculated according to the ASTM E1921-97 procedure. Moreover, finite element simulation of the SENB specimens, coupled with the applications of cohesive zone model (CZM), virtual crack closure technique (VCCT), the X70 pipeline steel’s critical energy release rate (ERR) is achieved and applied to predict the F C i of arbitrary specimen thickness while crack initiates, corresponding fracture toughness K C i are obtained and compared with the experimental ones. The present research will be beneficial for the prediction of pipeline steel’s actual fracture toughness and the retrenchment of experimental costs.