The atomic structure and structural stabilities of silicon oxycarbide SiO x C y phase formed at the SiC/SiO 2 interface were studied using ab initio pseudopotential method. The total energies and the cohesive energies of the SiO x C y at different composition: SiC/3 were calculated at zero temperature by relaxing the local atomic positions. The SiO x C y phase showed a structural preference to the SiC-like structure when carbon/oxygen (C/O) composition ratio was greater than unity (i.e., y > x), and to the SiO 2 -like structure when the ratio was less than unity (i.e., y < x). The charge distribution on atoms (Si, C and O) were calculated and it was found that electrons were transferred from Si to C with less amount than to O.