Abstract： Using the Green's function method with a tight binding Hamiltonian, the electronic structures of the ground states of S0/Se0, S0/Te0 and Se0/Te0 mixed pairs in Si are investigated. Two A1 symmetrical states are introduced by the mixed pairs, the bonding A1 state is higher than the antibonding A1 state. The numerical results of the energy leveals of mixed pairs are obtained, which are in agreement with the experimental observations. It looks likely that the unidentified shallower energy leveals (S0/Se0 (X1), S0/Te0 (X1), Se0/Te0 (X1)…) are not introduced by the mixed pairs with other non-nearest positions. We note that the transfer direction of the s wavefuction between two different defect atoms in Si is reversed as compare with the ordinary diatomic molecule case. The physical reason is discussed.