Fine Ni particles are effective in catalyzing hydrogen sorption of MgH2, but there is confusion about the extent of this effect in relation to Ni particle size and content. Here, effects of Ni particles of different sizes on hydrogen desorption of MgH2 were comparatively investigated. MgH2 mixed with only 2 at% of fine Ni particles rapidly desorbs hydrogen up to 6.5 wt% around 200-340 °C, but there is no significant difference in the desorption temperature of the mixture when Ni particles vary from 90 to 200 nm. Increasing the content of Ni to 4 at%, or a combined (2 at% Ni + 2 at% Fe), leads to hydrogen desorption starting from 160 °C. Further analyses of the literature suggest that the effectiveness of Ni catalysis largely depends on its site density over MgH2 surface, i.e., an optimal site density of catalytic particles is important in balancing the sorption properties of MgH2. The projected trend suggests that MgH2 can desorb hydrogen from 100 °C, the targeted temperature for fuel cells, if the number of catalyst sites is around 4 × 1014 per m2 of MgH2, or the number ratio of Ni to MgH2 particles is about a million to one.