Using ³⁰Si-implanted ²⁸SiO₂ and ^natSiO₂/²⁸SiO₂ structures, Si self-diffusion in SiO₂ was studied as a function of temperature and SiO₂ thickness (200−650 nm). Si self-diffusivity increased by about one order of magnitude with decreasing SiO₂ thickness from 650 to 200 nm with a SiN layer, i.e., the shorter the distance between the ³⁰Si diffusers and the Si/SiO₂ interface became, the higher Si self-diffusivity became. The dependence of Si self-diffusion in SiO₂ on the distance is caused by SiO generated at the Si/SiO₂ interface and diffusing into SiO₂. Si self-diffusion in SiO₂ was modeled taking into account the effect of SiO molecules. The simulated results showed good agreement with the experimental profiles. Furthermore, the simulation predicts that the self-diffusivity would increase for a longer annealing time because more SiO molecules should be arriving from the interface. Such time-dependent diffusivity was indeed found in our follow-up experiments, and the experimental profiles were also fitted by the simulation using a single set of parameters.