A novel process of using phthalimide to modify cellulose nanofibers (CNF) for CO2 adsorption was studied. The effectiveness of the modification was confirmed by ATR-IR. Phthalimide incorporation onto CNF was confirmed with the characteristic peaks of NH2, C–N, and ester bonding COO− was observable. The XPS analyses confirmed the presence of N1s peak in Ph-CNF, meaning that the hydroxyl groups reacted with the amino groups (NH2) of phthalimide on the CNF surface. Based on the results, surface modification and addition of phthalimide increased the specific surface area, but also decreased the overall porosity, size of pores and volume of pores. When the temperature, humidity, pressure, and airflow rate increased, the CO2 adsorption significantly increased. The CO2 adsorption of phthalimide-modified CNF was confirmed by ATR-IR spectroscopy as the characteristic peaks of HCO−3,NH+3 and ester bonding NCOO− were visible on the spectra.