Based on ab initio calculations,it is found that the donor center of substitutional sulfur (S) in diamond with C2v symmetry is more stable than that with C3v symmetry,which is different from previous reports in literature.The energy difference of C2v and C3v structures is qualitatively affected by the supercell size,and the 216-atom supercell could be proposed as the minimum to obtain stable configuration of substitutional S in diamond.Using supercells of up to 512 atoms,the donor level of substitutional S with C2v symmetry is deep.