The 1999 Chi-Chi earthquake in Taiwan represented the island's largest inland seismic event of the last century. This earthquake severely struck the western-middle part of the island. Abundant high-resolution digital seismic data were recorded by two seismic networks operated by the Central Weather Bureau (CWB). In this paper, we combined data from two CWB seismic networks to analyze the characteristics of the Chi-Chi earthquake sequence. The mainshock was relocated to 23.853°N, 120.816°E at the depth of 8 km. The focal mechanism was of a thrust type with strike 5°, dip 34° and rake 65° based on the new location and the first motion polarities. The distribution of the aftershocks revealed a special distribution pattern which marked out an aseismic dipping plane with the aftershocks mostly occurring on its upper side. It is highly suggested that there is a close relationship between this aseismic dipping plane and the Chelungpu fault.