The group velocities of Rayleigh waves along twenty-three paths, which pass through most major physiographic features of the Philippine Sea, are determined using the phase-matched filter technique. These paths are grouped into five different areas in each of which a major physiographic feature is present. Shear velocity structures of the upper mantle beneath the Philippine Sea are derived by inversion of the averaged dispersion data for each path group. The results suggest a substantially thinner lithosphere (about 30 km thick) and a much softer asthenosphere (with shear velocity as low as 3.8 km/sec) for this basin compared to typical oceanic structures. The derived models show that in the eastern volcanic islands, the softer layer is just underneath the lithosphere and its depth increases gradually to the west throughout the basin. This feature would not only reflect the high values of heat flow observations in the eastern area, but a. gradually cooling and solidifying feature towards the west may also proposed in the upper asthenosphere. However, variation of structures between the northern and southern portions of the West Philippine Basin is not easily distinguished in this current study.