Abstract: The uptake of radionuclide on kaolinite has been investigated extensively due to its low penetrability and high sorption capacity. Herein, kaolinite was characterized by acid-base titration. The uptake behavior of ⁶³Ni(Ⅱ) on kaolinite as a function of pH and ionic strength was studied by using batch technique under ambient conditions. The experimental data of ⁶³Ni(Ⅱ) sorption on kaolinite was obtained using the diffuse layer model combined with the FITEQL 4.0 program and the consecutive acidity constants as pK_a were optimized simultaneously. The uptake of ⁶³Ni(Ⅱ) on kaolinite contains ≡XONi⁺ species at low pH values and ≡YONi⁺ species at high pH values. Macroscopic experiment results show that the uptake of ⁶³Ni(Ⅱ) is greatly dependent on ionic strength at pH<8.5, and independent of ionic strength at pH>8.5. At low pH, the uptake of ⁶³Ni(Ⅱ) is predominated by ion exchange and/or outer-sphere surface complexation with Al³⁺/H⁺ on kaolinite surface, whereas inner-sphere surface complexation is the primary uptake mechanism at high pH-values. The macroscopic uptake data of ⁶³Ni(Ⅱ) surface speciation at the kaolinite/water interface should be significant to evaluate and predict the migration and uptake behavior of ⁶³Ni(Ⅱ) in environment mediums.