HZr2(PO4)(3) was prepared by the thermal decomposition of NH4Zr2(PO4)(3) which was synthesized in advance by a hydrothermal reaction from a mixed solution of ZrOCl2, H3PO4 and H2C2O4. Mixtures of HZr2(PO4)(3) with various amounts of CsNO3 were treated at 700-1200degreesC, in order to investigate the immobilization of Cs ion. When a mixture of CsNO3/HZr2(PO4)(3) in a molar ratio of 0.36 was treated at 700degreesC, the main product was suggested to be CsZr2(PO4)(3) from XRD measurements. The leaching rate of Cs ion from this product was less than 10(-10)g . cm(-2) . day(-1) in 0.1 mol . 1(-1) HCl solution at 100degreesC, indicating that HZr2(PO4)(3) reacts with CsNO3 to give a stable Cs-immobilized product.