A microcantilever (MC) coated by Al-doped ZnO nanorods (AZNRs) was used for carbon monoxide (CO) detection at ambient temperature. Very high sensitivity (on the order of femtograms) was obtained, which is caused by the intense adsorption of CO molecules on the AZNR surface. After the samples had been prepared and characterized, the effects of the presence of water vapor on the CO detection were investigated. Although no response was obtained for AZNRs without the presence of water vapor (i.e. under a vacuum), high sensitivity was observed in the presence of water vapor (finite humidity). Regarding the above phenomena, we propose a model that is a replacement process of the water vapor molecules by the CO molecules on the AZNR surface and discuss it in detail.