The widely accepted Bagnold-type function for calculating threshold wind velocity or shear velocity was developed for dry sands, but surface moisture is an extremely important variable controlling the entrainment process of sands by wind because the tensile force between the water molecules and sand grains produces cohesion. Here we report detailed wind tunnel experimental results on the threshold shear velocity of moistened sand of different sizes. The results show that relative threshold shear velocity, which is the ratio of threshold shear velocity of sand in the moistened state to that in the dry state, is better related to moisture content than threshold shear velocity itself. Function, modified after the Bagnold equation has been developed to estimate the threshold shear velocities of moistened sands. For a given grain size, the threshold shear velocity is proportional to (1 + KM)1/2, where, M is the moisture content, and K is a coefficient depending on grain size.