Amorphous nickel catalysts were synthesized by reducing the nickel(II) species in an aqueous NaBH4/NH3BH3 solution with and without l-arginine. The nickel catalyst with l-arginine maintains relatively high activity for hydrolysis of NH3BH3 to generate a stoichiometric amount of hydrogen with the cycle number up to 11 (827 mL s−1 (mol-Ni)−1 at the 11th cycle with l-arginine = 35 mg), while the reaction rate in the presence of the bare nickel catalyst was relatively low through the cycle number up to 11 (232 mL s−1 (mol-Ni)−1 at the 11th cycle). After catalytic reaction, the nickel catalyst with l-arginine possesses the high dispersion (diameters of nickel nanoparticles <5 nm), while the agglomerate of nickel in the bare nickel catalyst is observed. The results indicate that l-arginine maintains the dispersion of nickel nanoparticles (diameters of nickel nanoparticles <10 nm), leading to higher activity against cycle tests than the bare nickel catalyst.