The behavior of mobile protons at the Si/SiO2 interface has drawn substantial interest since it was found to play the critical role in a proposed non-volatile memory device based on Si/SiO2/Si structures. We have investigated the bonding and diffusion properties of a proton at the interface by a first-principles local density cluster total energy approach. A 73-atom cluster model is used to simulate the Si/SiO2 interface that is constructed from beta -cristobalite SiO2 on the Si(0 0 1) surface. In agreement with previous calculations of protons in bulk Si, the Si-Si bond center is found to be a stable site for a proton on the Si side of the Si/SiO2 interface. On the SiO2 side, the proton is found to bond with O atoms. Furthermore, it is found that the binding energy of proton-oxygen bond decreases as the proton penetrates into the oxide. Thus, an energy well is formed which confines protons in silicon oxide region in a Si/SiO2/Si structure. The calculated diffusion barrier for proton hopping between the neighboring binding sites is 0.73 eV in LDA and 1.15 eV in GGA, in good agreement with the experimental activation energy.