In order to clarify the mechanisms of intergranular stress corrosion cracking of Alloy X750 in high temperature water, the authors have studied the relationship between the distribution of hydrogen and the microstructure of grain boundary precipitates by using electron micro autoradiographic technology. Two types of Alloy X750 with different heat treatment history were prepared. Which is sensitive to the stress corrosion cracking in high temperature water, which has incoherent precipitates other than M23C6 discretely exist at the grain boundaries. The other type has excellent resistance to the stress corrosion cracking and has coherent M23C6 precipitates which semi-continuously exist at the grain boundaries. Tritium and hydrogen were charged into these specimens at room temperature by cathodic charging method. After the charging, specimens attached by the sensitive film (monolayer of fine silver bromide grains film) were kept in a dark box for about 10 days. The distribution of Ag grains on the specimen was observed by electron probe microanalyzer and transmission electron microscope. For the specimens which are sensitive to the stress corrosion cracking resistance, the Ag grains existed on the grain boundaries, however, for the specimens which have excellent resistance to the stress corrosion cracking, the Ag grains were not recognized at the grain boundaries. From this evidence, it seems that hydrogen trapping behaviour at the grain boundaries affects the stress corrosion cracking resistance.