The effect of heat treatment temperature on the microstructure and precipitate at grain boundary of G3 alloy was investigated by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and transmission electron microscope (TEM). And then, the effects of the microstructure change and precipitate at grain boundary on the pitting resistance of G3 alloy were studied by immersion tests and electrochemical measurements. The results show that the pitting resistance of G3 alloy is mainly affected by the precipitates at grain boundary. The pitting resistance of G3 alloy is increased after annealing at low temperature (500 oC), due to fewer precipitates in G3 alloy and the improved microstructure uniformity. With the further increasing annealing temperature, there are a large number of precipitates formed at the grain boundary, which can increase the microstructure inhomogeneity of G3 alloy, and the stability of the passive film in poor Mo region becomes worse. It is prone to local active dissolution in poor Mo region, and the susceptibility to pitting of G3 alloy increases significantly.