Although numerous studies have been made on the contrast between subducting slabs and surrounding mantle, little is known about the internal structure of the oceanic lithosphere after it descends into the mantle at subduction zones. The velocity structure within the slab can be expected to be heterogeneous, based on thermal and petrological considerations. In this study, observed travel time residual data from a spatially dense seismic network above deep earthquakes in the Izu-Bonin region are compared with three-dimensional ray tracing calculations. The data are inconsistent with a homogeneous slab model and consistent with a heterogeneous slab model with regional velocity variations. The residual data can be explained by a model that has a velocity gradient within the slab. In the subducting Pacific plate, the velocity near the center of the slab is faster than that near the upper boundary, and gradually decreases toward the bottom of the plate. A model with a velocity decrease of 3%, as predicted by a thermal profile, explains the observed data. ¿ American Geophysical Union 1992