Abstract: According to BS 7448 standard and microscopic analysis method, thermal simulation technique was used to study the fracture toughness (CTOD) of the coarse grain zone in heat affected zone of X100 grade pipeline steel under different t_8/5. The results showed that CTOD value increased as the t_8/5 time increased, and microstructure was transformed from lath bainite to granular bainite. The coarse chain M-A component appeared at the grain boundary of the prior austenite att_8/5= 40 s, which caused a small decrease in fracture toughness. Whent_8/5 was 50 s, the fracture toughness continued to increase. However, the grains tended to coarsen as t_8/5 increased.