The effects of foreign impurity ions in the conduction plane on the beta"-Al2O3 lattice have been analyzed by molecular dynamics. As impurity ions, K and Ca ions were chosen and Na sites in the conduction plane were replaced with these ions. Results are summarized as follows : (1) The beta"-Al2O3 lattice expands perpendicular to the conduction plane when K ions are doped; (2) Ca ions do not contribute to expansions of the beta"-Al2O3 lattice; (3) both K and Ca ions reduce the mean square displacement of Na ions, which can be attributed to decreased Na ion diffusion.