[en] We research the adsorption geometries and electronic structures of pristine graphene (p-GR) and Li-doped graphene (Li-GR) before and after CO adsorption by first-principles. The adsorption energies E_a_d of CO on p-GR and Li-GR are calculated. The results demonstrate that E_a_d of CO on Li-GR is from −3.3eV to −3.5eV, meanwhile Q is up to 0.13e, which indicate that strong electrostatic attractions occur between CO and Li-GR, while CO is physically adsorbed on p-GR. The obvious accumulated charge in electron density difference and increasing carrier density suggest that the conductivity of Li-GR is improved considerably after CO adsorption. An adsorption mechanism is also proposed. Our results provide a path to achieving CO sensors with high performance. (paper)