Geometry, electronic structure, and diffusion properties of Na atom on the pristine and single vacancy-defected silicene sheet have been systematically investigated by using density functional theory with generalized gradient approximation. The most stable positions of Na atom on the perfect and the defected silicene sheet are determined, and four diffusion paths have been explored. The minimum diffusion barrier of Na atom is 0.25 eV on pristine silicene, and 1.44 eV on single vacancy-defected silicene sheet. The vacancy exhibits a more significant influence than the adsorption of the Na atom on the electronic structure of silicene. (C) 2015 Published by Elsevier B.V.