Due to their ultrahigh theoretical capacity, metal borohydrides are considered to be one of the most promising candidate hydrogen storage materials. Their application still suffers, however, from high operating temperature, sluggish kinetics, and poor reversibility. Designing nanostructures is an effective way of addressing these issues, but seeking suitable approaches remains a big challenge. Here, a space-confined solid-gas reaction to synthesize Mg(BH 4 ) 2 nanoparticles supported on grapheme is reported, which serves as the structural support for the dispersed Mg(BH 4 ) 2 nanoparticles. More notably, density functional theory calculations reveal that graphene could weaken both the MgH bonds of MgH 2 and BB bonds of B 2 H 6 , which could thermodynamically and kinetically facilitate the chemical transformation to synthesize Mg(BH 4 ) 2 with high purity. Because of the synergistic effects of both the significant reduction in particle size and the catalytic effect of graphene, an onset dehydrogenation temperature of ≈154 °C is observed for Mg(BH 4 ) 2 nanoparticles, and a complete dehydrogenation could be achieved at a temperature as low as 225 °C, with the formation of MgB 2 as the by-product. This work provides a new perspective to tailoring the thermodynamics and kinetics of chemical reactions toward the favorable synthesis of functional inorganic materials.