In this study, waste polyethylene terephthalate (PET) based activated carbon (AC) with different textural properties were prepared by adjusting the KOH mixing ratio. The prepared ACs were characterized by SEM, TEM, XRD, XPS and nitrogen adsorption analysis. From the characterization results, ACs showed a good correlation between the KOH mixing ratio and the surface area, pore volume and pore size, but there was no correlation with the surface chemical composition. In addition, the adsorption equilibrium amount of CO2 and N2 was measured under different temperature and pressure conditions using a volumetric adsorption apparatus. The adsorption amount of the target gas showed a good correlation with the ultramicropore volume (V<0.9  nm) rather than the surface area and total pore volume of ACs. The gas adsorption behaviors according to the pore structure were compared and analyzed by adsorption isotherm models, isosteric heat of adsorption, and adsorption energy distribution function (AED). The selectivity for CO2 calculated using the ideal solution theory (IAST) under flue gas (CO2 : N2 = 15 : 85) conditions suggests that the waste PET-based AC can be sufficiently used for the related CO2 adsorptive separation process.