Cation effect on DNA/DNA, RNA/RNA, and RNA/DNA stabilities was investigated quantitatively. Plots of predicted delta G degree 37 values vs. measured delta G degree 37 ones showed the same relationship for DNA/DNA, RNA/RNA, and RNA/DNA. The number of sodium ion concerned in a double helix formation was estimated, and the result indicated that the number of bound sodium ion may determine the duplex stability regardless of DNA/DNA, RNA/RNA, and RNA/DNA. Since these duplex stability can be predicted with the nearest-neighbor model, it is considered that the number of bound sodium ions is affected by the nearest-neighbor bases. The importance of the screening effect of the electrostatic repulsion by bound cations was also indicated for determining the duplex stability as well as the hydrogen bonding and stacking interactions.