The MS 8.0 great earthquake of 12 May 2008 which occurred in the YingxiuBeichuan fault of Longmen Mountain belongs to strikesliptype thrust. With an emphasis on the geomorphic features of the surface ruptures of the great earthquake in Longmen Mountain, the eastern margin of Tibetan Plateau, this paper defines the vertical and horizontal offsets of the YingxiuBeichuan fault by detail field geomorphic surveys. The results show that the surface rupture of the YingxiuBeichuan fault stretches to NEE direction (NE30°~50°) for 180~190 km from Yingxiu to the surface rupture of Beichuan. The average vertical and horizontal offsets are 2.9 m and 3.1 m, respectively, with the largest vertical and horizontal offsets of 6.2±0.1 m and 6.8±0.2 m in Leigu town. The ratio of thrusting and dextral slipping in the YingxiuBeichuan is 3∶1~1∶1, indicating that the surface ruptures are characterized with thrusting displacement slightly more than or equal to dextral strikeslip one. Based on the nearly NStrending fault, the surface rupture of the YingxiuBeichuan fault can be divided into two segments of large rupture and two segments of small rupture, with the former located at the southern section (Hongkou) and middle section (Leigu) of Yingxiu, Chuanbei county, and Dengjiaba area. We divide the process of seismic rupture into two stages based on the scratch preserved on the fracture planes: early thrusting and later oblique slip motion.