A series of ocean-bottom seismograph experiments were conducted to establish velocity models in the southwestern offshore area of Taiwan from 1992 to 1993. Although the newly established system is still in its early stages, the data collected make it possible to define a crustal structure in the vicinity of the northern Manila Trench. The arrivals of deep refraction with apparent velocity at about 8.1 km/sec help to define the depth of the Moho discontinuity which is about 12 km below eas level. A thin crust of merely 5 to 6 km in thickness underlying the sediments to the west of the trench in the study area suggests an oceanic origin of the crust there. The thickness of the sediments is maintained over 3 km throughout the area, but a thicker crust to the east of the trench than that to the west is demonstrated.