A series of novel aryloxide imidazolidin-2-imine bidentate neutral Ni(II) and Pd(II) complexes bearing five- and six-membered chelate ring structures were synthesized and characterized. X-ray diffraction analysis results revealed that all of the Ni(II) complexes (Ni1-Ni3) and Pd(II) complexes (Pd1 and Pd3) adopted an almost square-planar geometry. In the presence of various cocatalysts such as MAO, MMAO, Et2AlCl, and EtAlCl2, all of the Ni(II) and Pd(II) complexes exhibited remarkably high activities (up to 2.6 × 107 g of PNB (mol of M)-1 h-1) toward the addition polymerization of norbornene. These catalyst systems produced high-molecular-weight polynorbornene (PNB) with narrow molecular weight distribution, except for the insoluble PNB obtained with Pd1-Pd3/MAO systems. The Pd(II) complexes showed particularly good thermostability with a high activity of 1.56 × 107 g of PNB (mol of Pd)-1 h-1 even at 80 °C. These complexes are rare examples of neutral Ni(II) and Pd(II) complexes bearing aryloxide-functionalized imidazolidin-2-imine ligands in the field of olefin polymerization.