We investigated the carbon solubility into silica under high temperature condition. Mixtures of amorphous silica and graphite powder sealed in silica glass tubes were heated at 1300 °C. The a lattice parameter and unit cell volume of α–cristobalite obtained are slightly increased compared with that heated without graphite. The c lattice parameter, on the other hand, almost unchanged. There is no clear dependency of the lattice parameter variations on carbon content mixed as the starting material. After re–heating the samples under atmospheric oxygen, the a lattice parameter and unit cell volume were reduced from the previous values. These changes indicate the possibility that carbon certainly incorporated into the α–cristobalite was oxidized with the atmospheric oxygen. The ab–initio calculation of disiloxane molecule showed that with carbon substitution for the bridging oxygen the Si–C bond distance increases whereas the Si–C–Si bond angle decreases compared with the Si–O bond distance and Si–O–Si bond angle. Consequently, the distance between Si atoms increases with the carbon substitution for the bridging oxygen. Since the expansion of Si–Si distance contributes twice as much to the a and b–axes than the c–axis in the unit cell of α–cristobalite, the ab–initio simulation result supports the observation that a lattice parameter increases with the carbon substitution relative to the c lattice parameter. The study strongly suggests that under reduced and high temperature conditions carbon is substituted not for silicon but for oxygen in α–cristobalite structure.