Application of the interferometric method to four ERS2-SAR images acquired before and after the 1999 Chichi earthquake has allowed determination of the coseismic surface displacement in the footwall area of the Chelungpu fault. The interferometric results revealed a relative shortening in the round trip distance between the radar antenna and the ground of the footwall side of Chelungpu fault, during the earthquake. This shortening progressively increased from the west to the east and reaches the maximum amount of approximately 26 cm near the central segment of the Chelungpu fault. Our interferometric results have been precisely examined using a dense GPS network in the investigated area. We mapped the GPS coseismic measurements into the radar line of sight and implemented a forward simulation of SAR interferogram from this synthesized result to control our unwrapping performance. In this study, these two observations are compared with a 3-D dislocation model of the fault. Finally, a deformation analysis based on our interferometric result has indicated that a segment with irregular deformation behaviour can be distinguished in the footwall area of the Chelungpu fault. This segment may result from either the influence of inherited basement faults or the presence of a structural terrace that provide local opportunities for superficial deformation.