Si self-diffusion rates in Mg2SiO4 polycrystalline wadsleyite were measured at 18 GPa and 1430--1630¿C using an isotopic tracer (29Si) and secondary ion mass spectrometry. The volume diffusion coefficient (Dv) and grain-boundary diffusion coefficient (Dgb) were determined to be Dv = 3.44 ¿ 10-11 2/s> exp (-299 /RT) and δDgb = 1.14 ¿ 10-17 3/s> exp (-248 /RT), respectively. Si diffusion rates in wadsleyite are about 5 orders of magnitude slower than Mg-Fe interdiffusion rates at 1400¿C. Assuming that Si is the slowest diffusing species in wadsleyite, the geophysical model of the viscosity in the mantle transition zone can be explained by diffusion creep in wadsleyite for a grain size of about 0.5--5 mm. Some portions in cold subducting slabs, where the grain size reduces to less than 1 ¿m after the olivine-spinel transformation, become weaker than the surrounding mantle.