A Japanese natural mordenite was modified by acid washing as well as cation-exchange. Crystal structure, porosity, and active sites of the modified natural mordenites were characterized, and their CO2 adsorptivity were examined. Contraction as well as expansion of the mordenitic unit cell in the natural zeolite and distortion of zeolite structure by acid washing are plausibly shown by XRD, Long-term washing by nitric acid induces enlargement of micropore volume, increasing the micropore dimension but decreasing the large pore dimension over the mesoporous range. The degree of dehydration has an important role in CO2 adsorption, Metal ionic sites affect both the irreversible CO2 adsorption and the CO2 equilibrium adsorption at low concentration, Li+ ion-exchange brings about a great CO2 adsorption but reduces the CO2-surface interaction due to molecular size effect. Ca2+ ion-exchange has its main effect in the enhancement of CO2 desorption energy.