The as-prepared and purified multiwalled carbon nanotubes (MWNTs) with a mean outer diameter of 5.1 nm were employed for hydrogen storage measurements. Compared with the as-prepared sample, the purified one has a much higher gravimetric hydrogen storage capacity. Then the purified MWNTs were cold-compacted into a cylinder for volumetric hydrogen storage measurements. The gravimetric and volumetric hydrogen storage capacities of the bulk sample reach 6.3 wt % and 31.6 kg/m(3) at room temperature under an equilibrium pressure of 14.8 MPa, respectively. In addition, the microstructure including surface structure and pore texture of MWNTs were investigated by electron transmission microscopy, high-resolution electron transmission microscopy, cryogenic nitrogen adsorption, and X-ray photoelectron spectroscopy. It is considered that the structural modification may be determinant for the improvement of the hydrogen storage capacity of MWNTs.