Abstract: The susceptibility to reheat cracking due to the heat-affected zone has been determined by using of the Y_type groove S. R. crack resisting test and other methods; and thin foil specimens taken from this heat-affected zone have been analysed by using Scanning Transmission Electron Microscope with attached EDAX. This combined analysis include morphological observation of- microstructure and determination of structure and orientation relationship. According to the precipitation hardening and creep deformation theory, the cause of reheat cracking has been discussed. It has been suggested that reheat cracking will be able to be controled and prevented through. Postweld isothermal treatment which has brought about the fragmentation of mosaic-block in the grains and a change in carbides precipitation and their distribution. After different experiments of postweld isothermal treatment and being proved by other related teels, this way of prevention has been proved as effective.