SAR interferograms have successfully determined the detailed coseismic displacements in the footwall block of the Chelungpu fault during the Mw 7.6 Chi-Chi earthquake in Taiwan on September 21,1999 (local time). Based on the 3D dislocation model of faults, we infer the preliminary rupturing behavior - a segmented rupturing process - of the Chelungpu thrust from recent SAR interferograms. A key argument controlling the rupture of the Chelungpu thrust may be the recognized segments with different sliding depths of Chinshui Shale.