We discuss the experimental results of silicon and oxygen self-diffusion coefficients in forsterite and iron-bearing olivine from the perspective of defect chemistry. Silicon diffusion is dominated by VO ··-associated VSi″″, whereas oxygen diffusion is dominated by hopping of VO ·· under anhydrous conditions, and by (OH)O · under hydrous conditions. By considering the charge neutrality condition of [(OH)O ·] = 2[VMe″] in hydrous forsterite and iron-bearing olivine, we get D Si ∝ ( $$C_{{{\text{H}}_{2} {\text{O}}}}$$ C H 2 O )1/3 and D O ∝ ( $$C_{{{\text{H}}_{2} {\text{O}}}}$$ C H 2 O )0, which explains the experimental results of water effects on oxygen and silicon self-diffusion rates (Fei et al. in Nature 498:213–215, 2013; J Geophys Res 119:7598–7606, 2014). The $$C_{{{\text{H}}_{2} {\text{O}}}}$$ C H 2 O dependence of creep rate in the Earth’s mantle should be close to that given by Si and O self-diffusion coefficients obtained under water unsaturated conditions.