Based on multiple-scattering self-consistent-field method, we have investigated the electron structrue of Co. cluster (n= 3-13). Comparing the calculated ioniza-tion potentials for different geometries and electron configurations with the experimental values, we can study the appropriate electron configuration and geometric structure of Co.. From the occupation of local 3d valence electron in configuration, we found, generally speaking, that the average atomic magnetic moment of Co clusters decreases as the number of atoms increases. From the geometry of cobalt clusters, we also surmise the various formation of Co. cluster.