AbstractWe investigate the relationship between water content and the parameters in the growth kinetics of forsterite phase transformation, i.e., the pre‐exponential factor and the Helmholtz free energy of activation, by analyzing the growth parameters of forsterite for water content varying from about 0.08 to 0.5 per cent in weight. The results show that increase of water content only decreases the Helmholtz free energy of activation for growth, while the pre‐exponential factor of the classical growth rate equation has weak dependence on water content and is nearly a constant, of which the value corresponds to an interphase boundary with the thickness of one or a few layers of molecules. This result is consistent with the observation of quench experiments on olivine and rheological studies. Accordingly, taking this result as a constraint, we predict the depth of metastable forsterite with different water contents. The dynamic phase boundary of forsterite changes from several kilometers below the equilibrium boundary of forsterite phase transformation to almost the 660‐km discontinuity. Different from forsterite, mantle olivine has smaller metastability. Under the nominal dry condition, the dynamic phase boundary of mantle olivine is 20 km shallower than that of forsterite. Our new results are in accordance with a former statement that there might be observable extent of metastable olivine in subduction zones although it is difficult to reach 660‐discontinuity.