In 1957, we studied a lot of historical records of the Jan. 23 1556 Guanzhong earthquake (M ≥ 8). We found two interesting records in the epicenter area:About eight hours before the great earthquake, people felt a kind of ground rotational movement and dizzied.We pointed out that is a kind of foreshock phenomenon^[1]. In 1971, the author and Qin Baoyan studied this phenomenon and considered that it is a slow motion which is useful for earthquake prediction^[2]3 In this paper, we intend to discuss the cause of the ground rotation before great earthquake. Besides, we also intend to discuss a significance of block rotation in earth-quake prediction