To investigate the effects of specific porous microstructure of diatomite on the hydrogen storage properties of MgH2, a two-step preparation was carried out. The first step was decrepitation of MgH2 particle size during 10 h of milling. The second step was additional 1 h of milling with diatomite. The microstructure and phase composition of materials was characterized by X-ray diffraction, whereas the powder morphology and degree of additive dispersion were analyzed by scanning electron microscopy. The hydrogen desorption behaviour of nanocomposites was investigated by differential scanning calorimetry. The results show that addition of porous diatomite structure leads to decrease in desorption temperature, since there was no other effect that can have an influence on kinetics, such as formation of the metastable γ-phase, reduction of oxides to the native metal and/or homogeneous dispersion of the catalyst. This indicates that the microstructure of added material plays the main role in the enhancement of desorption properties of composites.