Nanostructured Mg(60)Ni(5)Co(m)X(35-m) (X = Co, B, Al, Cr, V, Pd and Cu) body centered cubic (BCC) alloys were synthesized by mechanical alloying method. These Mg-based alloys with different lattice parameters can show significantly different hydrogen absorption properties. The BCC alloys with lattice parameter in the range of 0.300 similar to 0.308 nm absorb large amount of hydrogen at 373 K and the BCC alloys with the parameter larger than 0.313 nm have difficulty to absorb hydrogen at this temperature. Geometric effect is thought to be one of the dominant factors to affect the hydrogen absorption property of interstitial alloys. Nanostructure, fresh surface area and defects produced during mechanical alloying process are also important facts that make Mg-based alloys absorb hydrogen at 373 K. (c) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.