The (Ce,Sm)/Zr-pillared montmorillonite clay which is thermally stable at 300℃ can be obtained from ZrOCl2·8H2O solution by adding (Ce,Sm) cations, and d001 values are 2.1～2.3 nm at room temperature, 1.8～1.9 nm at 300℃. FTIR analysis shows that relatively strong chemical bonds of (［Si—O］—［H(O)—Zr(Ce,Sm)］) are formed between (Ce,Sm)/Zr polycations and Si—O tetrahedral sheets of the clay minerals. It is therefore held that (Ce,Sm)/Zr-pillared montmorillonite has a stable structure.