The 2003 Chengkung earthquake (Mw 6.8) provided diagnostic evidence for a source model showing the deformation process of the seismogenic Chihshang fault in eastern Taiwan. The aftershocks show a fault-bend at a depth of 18 km. Coseismic ground displacements recorded by strong-motion records allow us to deduce instant rupturing of this event. Our resulting model shows a fault length of ~33 km and dip-slip dominant rupture on fault-plane deeper than 18 km. Estimated coseismic displacements constrain two fault planes: one at 5--18 km depth dipping 60¿E and 18--36 km depth dipping 45¿E. The uppermost fault-plane of the Chihshang Fault (0--5 km) did not break immediately after the main shock; however, it may have a major role in after-slip and even interseismic ground deformation. The Taiyuan basin developed in the hanging wall is a geomorphic feature consistent with and adequately explained by coseismic ground displacements.