We report the synthesis, characterization, and solution self-assembly of plenary Nb6P2W12-based transition-metal substituted polyoxometalate, which is obtained by simply adding transition metals (Co2+) into aqueous solution containing cluster [(NbO2)(6)P2W12O56](12-) , which is obtained by an in situ synthetic method. The incorporation of Co2+ ions significantly affects the crystal structure, resulting in the formation of a 1D chain-like crystal and the first example of a niobotungstate-based cobalt derivative cluster. The behavior and stability of this cluster in solution are confirmed by time-resolved static light scattering, dynamic light scattering, small-angle X-ray scattering, and electrospray mass spectrometry studies.