Taiwan is located in a complex, tectonically active region where the Philippine Sea. plate is subducting northwestward along the Ryukyu Trench while the Eurasian plate is subducting eastward along the Manila Trench. The Central Weather Bureau Seismographic Network (CWBSN), which is composed of 75 stations with three-component instruments, has been operated over the island of Taiwan since March of 1991. This dense seismic network provides high-quality P- and S-wave arrival times of local earthquakes, and, thus, gives us a good opportunity to determine the velocity structure of the crust and upper mantle structures beneath Taiwan. We have applied a new tomographic method to infer the fine 3-D P- and S-wave velocity structures beneath Taiwan. Results show the existence of a low velocity zone in the uppercrust of west Taiwan. This zone is consistent with the thick sediments in the area. A low-velocity zone has been inferred underneath the extinct volcano regions, and it is due to the remnant geothermal effect after past volcanic activities. A low-velocity zone with a relatively homogeneous distribution of P-wave velocities, extending to a depth of about 30-35 km, is found beneath the Central Range. We infer that this feature might have resulted from the heat intrusion from the oceanic upper mantle of the Philippine Sea plate, which has been colliding with the Eurasian plate, and the effect of partial melting. The thickness of the crust of Taiwan is estimated to be 30-35 km and that for the Philippine Sea plate is 15-20 km.