In order to evaluate the selectivity for specific cations in ion-exchange of zeolites, we calculate the ion-exchange isotherms in various zeolites using Monte Carlo simulation techniques. The calculation results agree well with experiments. Furthermore, we examine the dependence of the cation selectivity on the Si/Al ratio and find that the Cs selectivity initially increases with the Si/Al ratio but saturates above a certain value. These results reveal that the selectivity for Cs is enhanced cooperatively by the microporous structure and the Si/Al ratio of the zeolite framework. The present simulation scheme is promising in selecting useful materials for Cs decontamination in waste treatments.