Potassium hydroxide (KOH) was employed to modify multi-walled carbon nanotubes (MWNTs), which were annealed at high temperatures to evaluate the properties of hydrogen storage in this study. Experimental results revealed that the structure of carbon nanotubes (CNTs) became destructive after being activated by KOH at 823 K in H-2 atmosphere. It was also found that the capacity of hydrogen storage onto the unmodified and KOH modified CNTs were 0.71 and 4.47 wt%, respectively, under ambient pressure and at moderate temperature. In addition to alkali-metal, both the residual metal such as cobalt and the defect structure were also considered as important keys to be used to improve hydrogen adsorption in the CNTs. The cause of which suggested that residual metal such as cobalt may be acting to dissociate hydrogen molecule into its atom state to spread out to the surface and inner layer through defect sites of the CNTs. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.