Phase relations in harzburgite have been determined between 14 and 24 GPa and 1473 and 1673 K. At 1673 K, harzburgite transformed to wadsleyite + garnet + clinopyroxene below 19 GPa and decomposed into an assemblage of ringwoodite + garnet + stishovite above 20 GPa. Certain amounts of akimotoite were produced at still higher pressures (22–23 GPa). Finally, perovskite and magnesiowüstite were found to coexist with garnet at 24.2 GPa. Compositions of all the phases were analyzed and elemental partitioning coefficients were determined among coexisting phases. Combining our experimental data with available thermoelastic properties of major minerals in the earth's mantle, we modeled the velocity and density signatures of the stagnated oceanic slab in the mantle transition zone (MTZ) under eastern China, based on kinematic slab thermal structure analysis. We examined two end-member slab models: a conventional straight slab with deformation thickening and an undulate slab with an oscillating wavelength of 200 km. We found that an undulated (buckled) slab model yields velocity anomalies (about 1–2% for Vp) that are consistent with seismic tomography models, taking into account low-pass filtering effects in seismic tomography studies. On the other hand, straight slab models yield velocity anomalies that are too high compared with seismic tomography models. Our models provide important constraints on the thermal structure, mineralogy, composition, density, and velocities of slab materials in the MTZ.