Seismic refraction data from onshore and offshore experiments in the eastern-northeastern Taiwan region were used to study the velocity structure by the two-dimensional ray-tracing method. In the velocity model, a structural fault boundary located beneath the Longitudinal Valley was used to separate the northern Coastal Range (CR) on the eastern side from the eastern flank of the Central Range (EFCR) on the western side. The P-wave velocities form the surface to the depth of 12-15 km varied from 3.9 to 5.8 km/s beneath the CR and from 4.8 to 6.1 km/s beneath the EFCR. Comparing the velocity structures along various latitudes, it was found that the CR extends northward to 34.2°N. The velocity structures of the CR, the Hsinchen Ridge (HR) and the Yaeyama Ridge (YR) indicate that the HR and the YR both belong to the same type of tectonic unit as the CR. To the north of 24.2°N, the velocity structure of the Ilan Ridge (IR), located between the EFCR and the EFCR and the southwestern end of the Ryukyu arc, is similar to that of the EFCR; hence, probably indicating it is the northeastern extension of the EFCR. This suggests that the EFCR bends eastward and belongs to the same tectonic unit as the southwestern Ryukyu arc. From a comparison of the velocity structures of the CR, EFCR and of other typical continental arcs, orogens and oceanic arcs in the literature, it can be concluded that the northern CR belongs to an oceanic arc and that the EFCR is a continental arc. Further more, form the analysis of the velocity structures beneath the CR and EFCR, it is believed that the upper crust of the CR is weaker in strength than the EFCR, which means that the arc-continent collision is not an appropriate model for the formation of Taiwan island.