We present a theoretical study of electronic structures and shape resonances of diatomic molecule C2 and C2+ and C2- Shape resonances are essentially quasi-bound states which are intimately connected with antibonding molecular orbitals. It is found that as the number of electrons decreases, the energy positions of the shape resonances shift downward, and as the corresponding antibonding molecular orbitals lie below the ionization threshold, the shape resonances disappear.