3D VP and VP/VS models for the source area before and after the Chi-Chi earthquake are derived from a tomographic inversion of seismic travel-time data from the Taiwan Central Weather Bureau Seismic Network (CWBSN). Resulting 3D velocity models reveal a strong lateral variation in the crust, particularly at a depth range of 5 to 25 km along the trend of the dipping Chelungpu thrust. In general, low VP and high VP/VS anomalies are found in and around the source volume of the Chi-Chi earthquake, a volume centered around the entire 100 km × 40 km Chelungpu rupture surface. We pay special attention to the crustal properties and their changes due to the Chi-Chi event and the association of the crustal anomalies with seismicity. Combining the results with those of other geological and geophysical surveys, the crustal anomalies are interpreted as images of a highly fractured and fluid-filled Chi-Chi source region. Fluid overpressure is known to reduce the strength of the fault zone and in this case may have initiated the nucleation of the Chi-Chi earthquake. The VP models obtained before and after the Chi-Chi event show some interesting heterogeneities. Regions of low VP anomaly appear to have further expanded with the occurrence of the Chi-Chi earthquake. The VP/VS model shows high anomalies near the source rupture, especially near where the rupture is believed to have nucleated. After the Chi-Chi event, the VP/VS model shows a substantial change that may reflect an outward shift of the fluid-filled fractured source region. This shift is shown by a general expansion in VP/VS anomalies to a broader region (e.g., where two intensive aftershock clusters occurred) and a significant VP/VS reduction to more normal values near the Chelungpu rupture. We attribute these changes in VP and VP/VS anomalies, as well as the expansion and shift of the anomalous regions, to be a response to the localized stress change caused by the slip of the Chi-Chi rupture. The location of the seismic events is correlated with that of the crustal VP and VP/VS anomalies. Large events appear to occur in the transition zones of both VP and VP/VS anomalies. This finding may be of use in the identification of weakened crustal regions in a seismic zone, thus suggesting where impending large earthquakes are likely to occur. All input data and initial and final models of this article are given as electronic files in the CD attached to the end of this issue.