The Sichuan Province in China is located in the middle section of a large north-south seismicity belt. The Xianshuihe Fault striking northwest-southeast, the Longmenshan east fault striking north-south, and the Anninghe Fault striking north-south together form a “Y” shape. Sichuan is located in the east margin of the Qinghai-Tibet crustal block. More than 8,000 earthquakes with magnitude 0.0 occur in Sichuan every year, and 2,000 of these can be located (Han and Lu, 2001). Since 26 B.C., 240 strong earthquakes have occurred in Sichuan with magnitude ML ≥ 5.0. Fifty of these events had ML 6.0-6.9, and 19 of them had ML 7.0-7.9. These events are distributed mainly along several faults and form seismic zones such as the Xianshuihe, the Songpan-Longmenshan, the Anninghe-Zemuhe, the Jinshajiang, and the Mingshan-Mabian-Zhaotong seismic zones (Han and Lu, 2001). A map of earthquake epicenters in Sichuan Province for events with magnitude greater than 2.5 during the period 1970 through 30 April 2003 is shown in Figure 1. In general, seismic epicenters in Sichuan Province show a diffuse spatial pattern rather than sharp linear alignment. The complex tectonic structure of the region, with crustal thickness varying between about 40 and 65 km (Ren et al. , 1987; Beghoul et al. , 1993; Vincent, 2000) and great sediment thickness over the Sichuan basin, creates a three-dimensional seismic velocity structure that affects the calculated locations. In this study we selected from a group of 1D velocity models the one that best fit the travel times of three ground-truth (GT) events from Sichuan, China. Based on this 1D velocity model, a set of station corrections for 106 stations in Sichuan area was generated. For the purpose of locating earthquakes quickly and within the desired accuracy, we modified the software programs Hypoinverse (Hypo) …