On 17 July 1998, an earthquake of magnitude 6.2 (ML) occurred near Chiayi, southwestern Taiwan. Two days after the occurrence of the mainshock, a temporary digital seismic network was deployed near its epicenter for 24 days to monitor the aftershock activity. The events within and near the network were first located using a traditional single-event location and then relocated using a joint hypocentral determination (JHD) technique to determine the significance of lateral velocity variations and to improve earthquake locations. The station corrections obtained from the JHD analysis very form 0.30 to -0.18 sec for the P-waves and from 0.51 to -0.58 sec for the S-waves. The patterns of station corrections suggest that upper crustal velocities on the eastern side of the study area are relatively higher than those on the western side. The depth cross-section of the relocated aftershocks shows a clear pattern of southeast-dipping distribution of the hypocenters. The focal mechanisms and hypocnetral distribution of the relocated aftershocks reveal a close relationship between seismicity and the known faults in the study area.