Abstract: In order to investigate the fracture toughness, crack tip opening displacement (CTOD) experiments were conducted on two X70 pipeline steel plates with different rolling processes. After the experiments, optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to observe the microstructure and fracture morphology. The effects of precipitates on the fracture toughness and the crack initiation mechanism induced by inclusions were analyzed. The CTOD result shows that the steel with a lower finishing cooling temperature has a higher fracture toughness. Inclusions with different shapes and two kinds of precipitates with different sizes were observed. It can be concluded that precipitates with different sizes have different effects and mechanisms on the fracture toughness. Distinguished from the earlier researches, inclusions enriched in silicon can be also served as the crack initiation.