Six ocean bottom seismographs (OBSs) were deployed along an east-west seismic line at 22 12¡¦N latitude in the eastern offshore region of southern Taiwan to record air-gun shots over the instruments. The line extends from 122 00¡¦E in the Philippine Sea Basin to 120 55¡¦E near the Hengchun Peninsula and crossed the North Luzon Arc, the North Luzon Trough, the Huatung Ridge and the Southern Longitudinal Trough, form east to west. A seismic velocity model along the profile derived from the azquired data includes two layers of sediments with a P-wave velocity of less than 3.6 km/s and upper and lower crusts with velocities of up to 5.3 km/s and 7.0 km/s, respectively. The sediment layer is very thin on the North Luzon Arc and Trough, but is as thick as 5 km under the Huatung Ridge and the Southern Longitudinal Trough. The crust, which is about 6 km thick under the Philippine Sea Basin, thickness westward and is more than 15 km under the North Luzon Arc and again towards the Hengchun Peninsula, with corresponding variations in Moho depth. The seismic structure in the area is thus consistent with a hypothesis that once a trench located along the Southern Longitudinal Trough might retreat farther west at a later stage.