Abstract: Using charpy-size three-point bending specimens with side-grooves, the authors evaluated the fracture toughness of HAZ of WT-62CF steel subjected to different thermal prestrains (e=4%, 6%, 8%, 11%) in two cooling temperature ranges on Gleeble-l500. In the 800~500℃ cooling temperature range, the values of Jm were respectively 10.84kg/mm(e=4%),11.65kg/mm(e=6%) and 10.96kg/mm(e=8%). In the 400~200℃ range, the values of Jm decreased continuously from 11.53kg/mm to 11.1, 102, 9.51kg/mm with the increasing of e from 4% to 6, 8, 11%. This was due to the effect of M-A constituent in the granular Bainite, which was the microstructure of simulated HAZ. The area fraction of M-A changed also with the thermal prestrains in a similar regularity to the fracture toughness. The disperse phase M-A is of benifit to the fracture toughness of Bainite.