SAR interferograms have successfully determined the detailed coseismic displacements in the footwall block of the Chelungpu fault during the M_W 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. Akey argument controlling the rupture of the Chelunhpu thrust may be the recognized segments with different sliding depths of Chinshui Shale.