Abstract: The coarne-grained zone in the HAZ of a new high strength steel weld was studied using thermally simulated specimens with an experienced thermal cycle representative of that zone. The structure and property of the specimens were then studied by means of SEM, TEM, as well as electron and X-ray diffraction techniques.It was found that with the increase of cooling time from 800℃ to 500℃(t_8/5),the impact toughness(a_k)of the specimens decreases.When t_8/5<180s, the deteriordtion of a_k is caused with the increase of the carbon-enriched M-A constituent content;and when t_8/5>180s, the coarsening of ferrite laths in the bainitic structure is responsible for the degradation of a_k. It was also found that the fine structure of the M-A constituent varies with t_8/5。