The nitric acid hydrothermal treatment (NAHT) of activated carbon (AC) was performed to greatly improve its activity of NO catalytic oxidation (NOCO) at room temperature by simultaneously altering pore size and N-doping. The activity was increased from 30.6% to 56.6% with the optimal nitric acid amount of 1.6 mL. Under this condition, the catalyst exhibited a more than 400 h stability, which is promising for a continuous NO removal. Characterization including SEM, TEM, XRD, Raman spectroscopy, N₂ adsorption/desorption, elemental analysis, H₂-TPR, FT-IR, in-situ DRIFTS, and XPS revealed that micropore distribution in 0.6–0.7 nm was increased and N-containing functionalities like pyridinic, pyrrolic and quaternary N were introduced to AC surface by NAHT, both of which contributed to the promotion of NOCO activity. The mechanism of NOCO by AC after NAHT was proposed, including the nanoreactor effect of micropores and NO adsorption controlled by surface basicity.