The electronic and magnetic properties of C-doped α-Al2O3 have been investigated using density functional theory calculations. It is found that magnetism can be induced by doping C at Al and O sites, in which the spin densities are mainly located on the C dopant. Doping C at the Al-site leads to band gap narrowing and greatly modifies the upper valance band, while doping C at the O-site affects the band edges of α-Al2O3 slightly. Based on the dipole selection rules, we have also assigned the sub-band electron transitions in both α-Al2O3:CAl and α-Al2O3:CO.