The boundary radii of cations are proposed and discussed. The periodic table of the boundary radii for one-valance cations is given, and the boundary radii for many ordinary high-valance cations are listed as well. The good relations of the cation boundary radii with Pauling, crystal, and Shannon-Prewitt cation radii are shown. This implies that the defined boundary radii for cations are reasonable and applicable.

Key words: IONIZATION ENERGY, IONIC RADII, PERIODIC CHART