Abstract: The annular crack defect occurs on the inner surface of the rolled seamless steel pipe as made of the large-sized 12Cr1MoVG CC round billet. The defect is observed by the metallographic microscope and the scanning electron microscope，and no large inclusions are found，the iron oxides as found in the crack are thick，and some are in granular form. High temperature inner oxidation points gathered in the crack defect，and decarburization is obvious on both sides of the crack. Thanks to the research via the pipe-rolling experiment it is identified that the annular crack defect on the inner surface of the steel pipe is caused by the V-shaped shrinkage hole in the core of the CC round billet during the piercing process. Therefore the said annular crack defect on the inner surface of the steel pipe can be eliminated by means of optimizing the process of the CC billet drawing speed，the electromagnetic stirring strength，the billet heating temperature and the piercing direction.