The theory of wet granular material is applied to the study of the influence of moisture on the entrainment of sand by wind in the first process of aeolian sand transport. The interparticle force due to water bridge is calculated using the toroidal approximation at first; and then the moment balance of a grain in the surface layer of sand bed is considered; finally, the change of threshold friction velocity with water content is obtained. (C) 2006 Elsevier B.V. All rights reserved.