Abstract: A possible mechanism of gravity anomaly before earthquake is put forward based on the up-down-up feature of gravity variation near the epicenter. Firstly, pore pressure in the dilatant region would drop down quickly during the medium -short term of earthquake preparation, and fluids around the dilatant region, especially the high-pressure and hot fluids from the deeper crust would intrude into it, hence cause the increase of gravity on the g round. Secondly, the ho t fluids would heat up the dilatant region gradually, subjoining an extensional stress to the shallow crust, which would decrease the density of the shallow crust. Thirdly, the process of fluids intruding into the extensional region of the shallow crust would make the gravity growup again. Some parameters of Tangshan and Zhangbei earthquake, such as density variation, the size of dilatant regionetc, are developed according to the observatory data before the earthquakes.

Key words: Upwelling of hot material, Before earthquake, Gravity anomaly