Abstract: The irradiation embrittlement mechanism of the RPV material is the base of the irradiation effect and the irradiation embrittlement prediction model of the RPV. The mechanism study is helpful for understanding the nature of the irradiation embrittlement, constituting the prediction model, and improving the technology for design and manufacture. It will be important to improve the performance of the RPV material. In this paper, the two stages of the irradiation embrittlement mechanism of the RPV material were stated in the way of the mechanism and the microstructure observing facility. The development direction was also discussed in this paper.