The 21 September 1999 Chi-Chi earthquake (M_L = 7.3) was the largest inland quake occurred in Taiwan in the past 100 years. According to direct field observations, a significant thrust rupture was occurred along the active Shuangtung fault and Chelungpu fault. From the integrated displacement derived from free-field strong motions around the source area, horizontal displacements over 9 m and vertical offsets reaching 3.4 m are obtained at the station TCU068 located nearby the northern section of Chelungpu fault. Based on the field observations of slip distribution over the entire 77-km surface break and the integrated displacement waveforms, we have arrived at the following findings: An obvious small offset at about 3 sec prior to the principal offset is observed at two sites close to the southern part of Shuangtung fault. This small offset is not shown on the other stations. We propose that a relatively small event, probably with a magnitude of about 6.0, occurred about 3 sec before the major rupture, that is located at the Shuangtung fault about 8 km east of the Chelungpu fault. The induced northward propagating rupture along the Chelungpu fault has reached a maximum permanent offset over 9 m at the northern section of fault. This northern section of the Chelungpu fault was identified as one of the heaviest damaged areas.