Based on first-principles plane wave calculations, we investigated the hydrogen adsorption on Pd-decorated single-walled carbon nanotubes (SWCNT). Our calculations revealed that the most hydrogen storage capacity is 2.88 wt.% with an average binding energy 292 meV for the uniformly Pd-decorated SWCNT, which is in good agreement with the experimental measurement. The electrostatic Coulomb attraction and the orbital repulsion mediate the interaction between H-2 and Pd. Our results predicted the binding energy and the hydrogen storage capacity can be further enhanced by using the open-shell transition metal/SWCNT media. (C) 2009 Elsevier B.V. All rights reserved.