The adsorption of Cs-137 and Sr-90 onto kaolinite in prescence of major groundwater cations (Ca2+, K+, Na+) with different concentrations was simulated by using triple-layer surface complexation model (TL-SCM). The site density (8.73 sites/nm2) of kaolinite used for TL-SCM was calculated from it's CEC and specific surface area. TL-SCM modeling results indicate that concentrations dependence on 137Cs and 90Sr adsorption onto kaolinite as a function of pH is best modeled as an outer-sphere surface reaction. This suggests that Cs+ and Sr2+ are adsorbed at the β-layer in kaolinite-water interface where the electrolytes, Nacl, KCl and CaCl2, bind. However, TL-SCM results on Sr adsorption show a discrepancy between batch data and fitting data in alkaline condition. This may be due to precipitation of SrCO3 and complexation such as SrOH+. Intrinsic reaction constants of ions obtained from model fit are as follows: Kintcs=10-2.10, KintSr=10-2.30, KintK=10-2.80, KintCa=10-3.10 and KintNa=10-3.32. The results are in the agreement with competition order among groundwater ions (K+〉Ca2+〉Na+) and sorption reference of nuclides (Cs-137〉Sr-90) at kaolinite-water interface showed in batch test.