Abstract: Plumb line variations (PLV) at the points with spacing of 0.1deg;times;0.1deg;in Tangshan region during 1987mdash;1998 are determined by using 46 batch repeated gravity observations of the Beijing-Tangshan network. Relation between underground material change (UMC) and PLV on ground are discussed and an approach of studying the UMC from regional PLV observation is developed. The UMC before and after two earthquakes (3 Aug. 1988,mb4.3; 5 Oct. 1995,mb5.0) in Tangshan region, as examples, is studied, giving the location and depth of the mass center of underground disturbed body, as well as the mass quantity. It has been demonstrated that the obtained UMC is responsible for the PLV,as well as the gravity changes in the area, where an earthquake may be preparing and is going to occur. It is also possible to study the UMC from the PLV result deduced from the repeated gravity observations in an area. UMC determination is expected to provide new message in earth science study, including earthquake prediction research.