The reduced Mulliken overlap population P mm′ was found to be a suitable indicator of the bonding strength between the metal—metal in the transition metal cluster compound in this paper. It was found that P mm′ can be the concentrated expression of the relation between the bonding strength and factors, such as the d-electron number of the metal atom, the nature of the bridge group and the ligand, according to the EHMO calculations and experimental results of the electron absorption spectra for compounds of the transition metal cluster.