The effects of thickness, notch orientation and delamination cracks on the impact toughness of X70 pipeline steel are investigated experimentally by use of the instrumented Charpy impact tests at different temperatures. The couple effect of delamination cracks, thickness, notch orientation and temperature is discovered. The delamination cracks have certain direction, and their amount and size are related to the temperature and the specimen thickness. Though the delaminating orientations of T-S and T-L specimen are not same, the reasons for both T-S and T-L specimen delaminating are that the weak interfaces in the specimens are pulled apart by the stress perpendicular to them. The delamination cracks can improve the actual impact toughness of X70 pipeline steel both T-L and T-S specimens. The effect of delamination cracks on the actual impact toughness changes with the thickness and the temperature. The couple effect of wall thickness, defect orientation and working temperature of pipeline must be taken into account in safe assessment of pipeline.