Abstract: Microstructures and properties in the intercritical reheated coarse-grained heat-affected zone (IRCGHAZ)of WB36 (15-NiCuMoNb5)steel were studied by means of thermal simulation.In particular, the forming of M-A constituent in the IRCGHAZ and its influence on the toughness were investigated.Experimental results show that the coarsing lath martensite (ML)is retained in the IRCGHAZ, however, a great deal of M-A constituents are formed in IRCGHAZ compared with the coarse grain heat-ffected zone (CGHAZ), with elongated M-A constituents distributed at ML boundaries within the prior austenite grain and necklace-like blocky M-A constituents along the prior austenite grain boundaries.The toughness of IRCGHAZ is the lowest compared with that of the CGHAZ, the second CGHAZ and SCCGHAZ, which led to the local brittleness zone in HAZ.The loss in impact value of the IRCGHAZ was due to the formation of elongatedM-A constituents in the interior of the grains, but not related to the formation of necklace-like M-A constituent along the prior austenite grain boundary region.