The decrease of photoconductivity from 1.3 to 0.25 mu A in ambient air or in water vapor atmosphere has been observed when the cobalt-doped zinc oxide nanobelts are irradiated with 630 nm light. This kind of negative photoconductivity is attributed to the photodesorption of water molecules from nanobelts' surface, and the mechanism was discussed. This result supplies a potential method to detect water vapor with a low concentration in environment. Crown Copyright (c) 2008 Published by Elsevier B. V. All rights reserved.