Abstract： The electronic structures of the ground states of S0 Se0 and Te0 pairs in Si are invesgated using the Green's function method with a tight binding Hamiltonian. Three different opinions are discussed. (S0)2, (Se0)2 or (Te0)2 in Si will introduce a symmetrical A1g state and an anti-symmetrical A2u state in the gap, both are fully occupied, the observed state is the shallower A1g state. The theoretical reason why the symmetrical A1g, state is higher than the antisymmetrical A2u state is analysed. The measured g factor of (Se2)+ in Si and the experimental data of ESR spectra for (S2)+ and (Se2)+ in Si also support the conclusions of the present paper.