As an attempt to investigate the sub-surface geometry of the Peikang High, and the structural features that may have influenced the regional seismicity of western Taiwan, a three-dimensional (3-D) P-wave velocity model has been developed. The velocity model was determined by simultaneously inverting of P- and S- wave arrival times recorded by the Central Weather Bureau Seismographic Network for earthquakes occurred from 1991 to 2000. In order to have a good constrain for the shallow part of the velocity model, the results of geophysical survey and drilling data of the Peikang High region conducted by the Chinese Petroleum Corporation are used in the initial 3-D model. Based on the results of restoring resolution and checkerboard test, the upper and middle crust is well revolved throughout the onshore and southern part of offshore in the study area. In general, the velocity transition zone from low to high is located beneath the Western Foothills region. The prominent feature of the velocity model is that the uplift high velocity volume exist in the middle- and lower-crust beneath the areas west to the Western Foothills should be associated with the Peikang High. The Peikang High is overlaid with the Coastal Plain, a basin type containing low velocity materials. The geometry of the Peikang High materials seems to affect the distribution of surficial faults and topography. In addition, most of the relocated Chi-Chi hypocenters are in the low velocity zones of the study area.