Properties of hydrogen physisorption in single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs) and SWCNT arrays are investigated in detail by grand canonical Monte Carlo simulation. The optimization of hydrogen storage capacity at 298 K and 10 MPa as a function of SWCNT diameter, MWCNT inter-tube spacing, and SWCNT array configuration is discussed.