Abstract: Studied and analyzed are the causes for the rather low impact value of the 12Cr1MoVG boiler pipe.With relevant research tests,it is identified that the contents of gases O and H are rather high,and that a great amount of tiny inclusions of Type B is dispersed in the base metal of the pipe.And energy spectrum analysis is also conducted on the pipe,the result of which shows that the inclusions are mainly composed of composite oxides like silicon oxide and aluminum oxide.It is just the rather high contents of O and H,and the existence of the said oxides that leads to increase of the brittle transition temperature,and the lessening of the impact value at the ambient temperature,which result in the brittle fracture.