Using travel time data form local earthquakes and air-gun shots recorded by the Central Weather Bureau Seismographic Network, the transition form a typical subduction to a collision suture in the southeastern Taiwan area is imaged in terms of a three-dimensional Vp structure. The southern prolongation of the Longitudinal Valley Fault (PLVF), which is characterized by a sharp contrast in velocity on either side, is the primary feature in the velocity structure. West of the PLVF, a high velocity volume exists from the surface to about 9-km in depth, which can be interpreted as being related to the Central Range. The Central Range Structure seems to end near 22.2°N beneath the Hengchun Peninsula. East of the PLVF, a major high velocity anomaly in the middle- to lower-crust beneath the Southern Longitudinal Trough and Huatung Ridge is ovserved. According to the velocity structure and the estimated composition, the high velocity body could be the forearc oceanic crust, which might have been torn off and separated from the Philippine Sea plate after the Luzon arc was formed, and has been shortened during the collision of the Eurasian and Phillippine Sea plates. The other conspicuous feature of the Vp model is a clearly lateral velocity variation across the Taitund Canyon form the surface to about 25-km in depth, which might be associated with the segmentation of the Luzon arc. Using the three-dimensional Vp model,earthquake events that occurred from 1990 to 1997 were relocated. Most of the relocated hypocenters in the study area tend to lie on the locations where there is a greater gradient in the Vp model.