Mg nanowires with the diameters of 30-50 nm, 80-100 nm, and 150-170 nm, which were prepared via a vapor-transport method, exhibited enhanced kinetics for hydrogen absorption/desorption. The present results clearly show that thinner Mg/MgH2 nanowires have a much lower desorption energy than that of thicker nanowires or bulk Mg/MgH2, indicating that changes in kinetics and thermodynamics are expected if the diameters of the nanowires are thinner than 30 nm.