In this first principles study based on density functional theory, we report the hydrogen storage capability of (5, 5) single walled carbon nanotubes coated with Lithium hydride and Nickel hydride. The paper brings out the role of lightweight Li atom and heavy Ni atom in binding the respective hydrides and hydrogen molecules with the single walled carbon nanotubes. The investigation is carried out for half and full coverage of the adsorbates (metal hydrides) on the sidewalls of the carbon nanotubes. The clustering of the adsorbates is observed in full coverage case of both the systems and its effect on hydrogen storage capacity and binding energy is reported. The clustering patterns are different in each of the systems and dependent on the nature of the metal atom in the metal hydride. The storage capacity of single walled carbon nanotubes coated with heavy transition metal hydride is around 3 wt.% whereas it is around 6 wt.% in their counterparts coated with lightweight metal hydride. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.