The effect of the SiO2/Si interface on Si self-diffusion in SiO2 during thermal oxidation was investigated using silicon isotopes. Samples with natSiO2/28Si heterostructures were oxidized at 1150 ~ 1250 °C and the 30Si diffusion in 28SiO2 during the thermal oxidation was investigated by secondary ion mass spectrometry (SIMS) measurements. Near the SiO2/Si interface, a significant profile broadening of the 30Si isotope from natSiO2 toward the newly grown 28SiO2 was observed. This 30Si self-diffusivity sharply decreases with oxidation time and hence with increasing distance between 30Si diffusion region and the interface. This distance-dependent 30Si self-diffusion was simulated taking into account the effect of Si species generated at the interface upon oxidation and diffusing into SiO2 to enhance Si self-diffusion. The simulation fits the SIMS profiles and these results indicate that Si species, most likely SiO, are emitted from the SiO2/Si interface upon Si thermal oxidation to release the oxidation-induced stress, as has been predicted by recent theoretical studies. Furthermore, combined with our recent results on O self-diffusion, the diffusion behavior of the emitted SiO near the SiO2/Si interface is discussed.