An alternative approach to the ionization potential of transition-metal clusters has been presented. The mean effective coordination number of surface atoms is taken as a parameter to reflect the d-band width of the cluster and a tight-binding theory is used to establish this relation. We have constructed a simple expression for the ionization potential and calculated the ionization potentials of the clusters ${\mathrm{Ni}}_{\mathit{n}}$ and ${\mathrm{Nb}}_{\mathit{n}}$, and have found that they agree quite well with the experimental results.