Abstract: Heating process of welding thermal cycle of 12% Cr stainless steel was reproduced with Gleeble 3800 thermo-mechanical simulators.Microstructure transformation,grain growth and precipitating phases were investigated with optical microscope,field emission scanning electronic microscope(FE-SEM) and transmission electron microscope.The following conclusions have been drawn.That the microstructure is almost fully austenite while the heating temperature is above 900℃.This austenite transforms to martensite during the cooling process and is retained down to room temperature.When heating temperature is higher than 1 250℃,the austenite is completely transformed to delta ferrite.The grains slightly grow when heating temperature is below 1 250℃.At the other hand,if the heating temperature is above 1 250℃,grains are dramatically coarsened with an increase of peak temperature and holding time.Grain growth is effectively inhibited by the existing second phase.There are large numbers of finely dispersed complex carbides containing Nb and Ti in base metal matrix.As heating temperature is up to 1350℃,these carbides are dissolved into matrix.Therefore,the pinning grain boundaries and inhibiting grain growth effects of fine particles disappear.