Prof. Hao Xiaoguang, Fang Jian, Liu Genyou and Hu Xiaogang published a new book recently, titled by Hypothesis of the Earth's Normal Density (Surveying Press). Two journals, Acta Geodaetica et Cartographica Sinica(vol.8 2009) and Bulletin of Surveying and Mapping(vol.4 2009), have reported the publication of the book. The authors put forward the concept of latitudinal normal density function of the Earth mantle. The Hypothesis of Earth's normal density is established. Gravity mechanism of the movement of the Earth's plate is discussed according to the hypothesis, which combines gravity theory with the plate movement. The effort of producing the book is sponsored by the National Natural Science Foundation of China and Key Laboratory Dynamic Geodesy of WHIGG. Congratulations!