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Global climate change or global warming due to carbon dioxide (CO2) emissions from humanity activities are believed to be one of the biggest challenges in twenty-first century, where more than 30 billion tons of this gas are released to the atmosphere annually. Among many technical procedures which can be employed to carbon capture from the flue gas stream after combustion, adsorption process utilizing advanced adsorbent materials such as highly porous Metal-Organic Frameworks (MOFs) has gained many attentions thank to many techno-economical and environmental benefits. In this paper, to enhance diffusion and adsorption capacity of CO2 inside two types of the most stable zeolitic imidazolate frameworks namely the ZIF-8 and ZIF-90, subset of MOF materials, ligand functionalization using various functional groups is accomplished through Density functional theory (DFT) and Grand Canonical Monte Carlo (GCMC) simulation. To this purpose, at first the structure of four hypothetical functionalized-ZIF-8 and -90 by NO2 (nitro group), NH2 (amino group), OH (hydroxyl group) and Li is designed using DFT calculations. Afterwards, CO2 adsorption capacity and diffusion in all functionalized-ZIFs are evaluated by GCMC simulation and molecular dynamic calculation, respectively. The accuracy of the calculated results is also evaluated by comparison with experimental data in literatures. Obtained results from simulation study revealed that in both ZIF-8 and -90 structures, Li functional group has the highest influence on the adsorption capacity for CO2 owing to high polarity of Li cations. It is believed that lithium ions create very strong negative charge centers at oxygen atoms that attack to the positive center of carbon in the CO2 molecule during adsorption.