The destructive Mw 7.6 Chi-Chi earthquake shook the entire island of Taiwan on 21 September, 1999 (20 September 17:47 at UTC). The earthquake was followed by tremendous amount of aftershocks for the next a few months including ten strong aftershocks with magnitude greater than 6.0 (Figure 1). North-south trending about 85 km long surface rupture with vertical thrust and left lateral strike-slip offsets with the maximum 9.5 meters displacement was charted by field investigations. Here we show high resolution P- and S-wave velocity models in the devastating 1999 Chi-Chi, Taiwan, earthquake source area and its vicinity to understand the generating mechanism of the main shock and aftershock sequence.