The widely accepted Bagnold type function to calculate threshold wind velocity or shear velocity was developed for dry sands. However, 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. The detailed results from experiments in a laboratory wind tunnel are reported, in which the threshold shear velocities for a range of sand grains with different moisture content are measured. The results show that the functions relating threshold shear velocity and moisture content are different for different sized sands. Relative threshold shear velocity, the ratio of threshold shear velocity of sand at moistened state to that at dry state is better related to moisture content. For given grain size, the threshold shear velocity is proportional to the square root of moisture content.