The Mw = 6.8 Chengkung earthquake on December 10, 2003 in eastern Taiwan was associated with abundant aftershocks. To explore the seismogenic mechanism and its causative fault system, we relocated the aftershocks, determined the focal mechanisms from P-wave polarities and used the Finite Dimension Source Model method to identify the fault plane. Based on our results, the earthquake can be divided into three seismic clusters: one is located in the east of the southern Longitudinal Valley, one on the Chihshang fault, and the other on the western side of the Luyeh fault to the south. Both the seismic distribution and Finite Dimension Source Model method show that east-dipping fault planes with thrust mechanisms coincide with previous studies based on background seismicity. The third seismic cluster suggests thrust mechanism along west-dipping fault plane that was not recognized by background seismicity. This seismic cluster occurred along the Central Range Fault previously proposed as active by geomorphic and geologic evidence.