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Greenhouse gas emissions have increased dramatically over the past years and had a significant impact on global warming. This study investigates the modification of multi-walled carbon nanotubes (MWCNTs) with diamine precursor to improve the carbon dioxide adsorption capacity. To achieve this goal, pristine multi-walled CNTs were functionalized in a two-step process. In the first step, multi-walled carbon nanotubes were functionalized with a mixture of diluted sulfuric and nitric acid (5 M HNO3/5 M H2SO4 with a volume ratio of 1:3) to sequestrate catalytic metal particles and oxidation of MWCNTs. In the second step, oxidized carbon nanotubes were functionalized with 1,3-diaminopropane (DAP) solution to improve the performance of multi-walled CNT in the carbon dioxide adsorption process. Specifications and characteristics of raw and modified carbon nano tubes were determined using FTIR, SEM, TGA, XRD, and N-2 adsorption-desorption isotherms at 77 K. The CO2 adsorption capacity was measured at 303-323 K and pressures up to 17.3 bar using volumetric method. At 303 K and pressure of 17.3 bar, 92.71 mg g(-1) of CO2 was adsorbed on MWCNT/DAP, while the CO2 uptake of raw MWCNT in similar conditions was just 48.49 mg g(-1). The results revealed that amine groups attached to the carbonaceous surfaces during the functionalization process cause the formation of carbon dioxide-adsorption sites on multi-walled CNTs which increased the adsorption capacity of MWCNTs. Experimental data was modeled with Langmuir and Freundlich adsorption isotherms and concluded that the Freundlich model has more fitness with the experimental data.