A highly sensitive (at the picogram order) Al-doped-ZnO nanorods (AZNRs)-coated resonant microcantilever (MC) for carbon monoxide (CO) gas sensing, operable at ambient temperature and under humid condition was synthesized. The ZnO nanorods were grown on the MC surface by using the hydrothermal method and doped by aluminum (Al). The sensitivities of un-doped and AZNRs for various CO gas concentrations were investigated by measuring the changes in the resonant frequencies and the Q-factor analysis of the vibrating MCs. The mechanisms of the remarkable enhancement of the sensitivity of the AZNRs-coated-microcantilever to CO were thoroughly investigated and discussed on the basis of energy gap between the highest occupied molecular orbital-lowest occupied molecular orbital of AZNRs and CO, active surface area of AZNRs, and the interaction of water vapor on AZNRs surface with CO. © 2020 The Japan Society of Applied Physics.