Abstract： The crystal structure and the atomic parameters of Li2K(IO3)3 and Li2NH4(IO3)3 have been determined by means of the X-ray powder diffraction method. The Li2K(IO3)3 and Li2NH4(IO3)3 are isomorphic with Li2Rb(IO3)3. The unit cell is monoclinic. The space group is P21/α. There are four formula units per unit cell. Lattice parameters are α= 11.198?, b = 11.046?, c = 8.254?, β= 111.53° and α =11.327?, b = 11.078?, c = 8.341?,β = 111.87° respectively. The relation between formation of Li2M(IO3)3 compounds and Mionic radius has been discussed.