The property of the hydrogen diffusivity in synthetic polycrystalline ringwoodite was studied by OH distribution in samples annealed under hydrothermal conditions. The experiments were conducted at 18 or 20 GPa and temperature range from 900 °C, to 1300 °C by Kawai-type multi-anvil apparatus. Diffusion in ringwoodite is about 1.5 orders of magnitude slower than in polycrystalline wadsleyite (Mg2SiO4) [Hae 2003], but it is about one order of magnitude faster than that along the c-axis (most rapidly) in single crystalline forsterite (Mg2SiO4) [Demoucy and Mackwell, 2003]. A magnitude of diffusion coefficients and activation energy obtained in our study implies that hydrogen diffusion distance is about 9.5 km in 108 years in the case of 1600°C and water is still heterogeneously distributed in the mantle transition zone. Therefore, it is thought that physical properties, such as the electrical conductivity and melting temperature, could vary in the transition zone depending on the geological settings.