Abstract: The influence of welding thermal cycle on toughness and microstructure in grain-coarsening region in the heat affected zone (HAZ) of X80 pipeline steel was investigated by weld thermal simulation technique, metallography, transmission electron microscopy (TEM), the instrumented Charpy impact toughness test and fracture toughness test. The results showed that there were different microstructures in the simulated grain-coarsening region of X80 pipeline steel under six different welding thermal cycle conditions. When the thermal cycling parameters were small,the microstructure of grain-coarsening region was primarily lower bainite and lath martensite, and when the thermal cycling parameters increased,it was mainly granular bainite. The structural morphology of M-A islands changed from thin and short bars to large long bars or large piece-like,and distributed in the grains instead of on the grain boundary. At the same time,the number of the M-A islands increased and the toughness got worse.