Abstract: ¹³⁷Cs could be solidificated in lattice of hydrothermal synthesized pollucite at relatively low temperature. The structures of pollucite might be greatly influenced by K⁺/Na⁺ coexisting with Cs⁺ in radioactive wastes because of the similar nature and properties of alkali metal ion. Based on above, pollucite micro-spheres were firstly hydrothermally synthesized by using metakaolin, nano-SiO₂ and CsOH•H₂O according to the stoichiometric composition of pollucite (CsAlSi₂O₆). Then, the effect of K⁺/Na⁺ on formation and structures of pollucite was researched through the substitution of CsOH•H₂O by NaOH or KOH. Results indicate that relatively low hydrothermal crystallization temperature of pollucite with polycrystalline nature is observed. Pollucite micro spheres would be the main products when Cs⁺ coexists with K⁺, and diameters of pollucite spheres increase with K⁺. However, K⁺ would exist in potassium aluminosilicate hydrates with low crystallinity instead of involving in the pollucite formation. Pollucite and analcite are the main products when Cs⁺ coexists with Na⁺. Na⁺ or Cs⁺ might be in structure of analcite and pollucite, respectively. It is found that the K⁺/Na⁺ could not enter and occupy the alkali ion sites in pollucite structure under hydrothermal conditions because of the non-migration and non-exchange of Cs⁺ in pollucite structure. However, hydrothermal crystallization temperature of pollucite could be further reduced and the crystal size of pollucite would increase in the presence of K⁺/Na⁺. Cs⁺ would be still stablely solidificated in the lattice of as-formed pollucite when K⁺/Na⁺ coexists with Cs⁺.