Catalytic activities of ruthenium complexes were examined in the metal-catalyzed living radical polymerization of methyl methacrylate (MMA) and in halogen exchange reactions between the dormant species and the ruthenium complex, along with cyclic voltammetry (CV). Among half-metallocene-type ruthenium(II) complexes, the activity increased in the order Ru(Cp)Cl(PPh3)(2) < Ru(Ind)C1(PPh3)(2) < Ru(Cp*)Cl(PPh3)(2) (Cp = eta(5)-C5H5, Ind = eta(5)-C9H7, CP* = eta(5)-C5Me5); namely, the lower the redox potential of the complex las measured by cyclic voltammetry), the faster the polymerization and the halogen exchange reaction. There were no apparent correlation, however, among the polymerization, the halogen exchange, and the redox potential for complexes of different structures such as Ru(Cp)Cl(PPh3)(2), RuCl2-(PPh3)(3), and Ru(Tp)Cl(PPh3)(2) [Tp = hydrotris(1-pyrazolyl)borate]. Thus, the catalytic activity of the complexes for the living radical polymerization can be predicted from the halogen exchange reaction or their redox potential in some cases.