Abstract: 3d metal-cyclopentadienyl sandwich compounds have been analyzed in detail by EHMO procedure. The results show that the chemical bond between cyclopentadienyl ligand and transition metal, Cp-M should involve three types of σ-, π-and δ-bond, in which π-bond is pricipal component and σ, δ-bond are mainly contributed by the metal orbitals. The effect of three bond types on the stability of these compounds was also discussed.