Water can play important roles for Earth's evolution. In this paper, we summarize recent studies on the effect of water on the phase relations, i.e., location of phase boundaries of the olivine-wadsleyite transition, post-spinel and post-garnet transitions, and that on kinetics of the phase transition of the olivine-wadsleyite transformation. Water expands the stability field of wadsleyite and ringwoodite. Water enhances the transformation kinetics from olivine to wadsleyite. The effects of water on the phase transitions can affect the topography of the 410 km and 660 km discontinuities, and dynamics of slabs descending into the transition zone and lower mantle.