Cu 2O particles are active catalysts for hydrogen evolution from ammonia borane (AB) by hydrolysis, however, Cu 2O particles easily form agglomerates as a result of highly reduced conditions during the reaction. In order to suppress agglomerate formation, capping of Cu 2O with organic reagents or inorganic materials was performed and the catalytic reactivity in AB hydrolysis was examined. Among the examined methods, capping of Cu 2O particles with Co 3O 4 nanoparticles was the most effective to avoid agglomerate formation of Cu 2O particles. The finding enabled us to examine the shape effect of Cu 2O particles on the catalytic reactivity in AB hydrolysis in the presence of Co 3O 4 nanoparticles. Comparisons of turnover frequencies for hydrogen evolution of Cu 2O-Co 3O 4 composites, in which Cu 2O particles were in the shape of 50-facets, cube, octahedron or rhombicuboctahedron, indicated that the composite with Cu 2O with the shape of 50-facets showed more than 7-fold higher hydrogen evolution rate normalized by surface area than the composite with Cu 2O with the octahedral shape. The size and shape effects of Co 3O 4 nanoparticles were also investigated on their ability to protect Cu 2O from agglomeration. Comparisons of the catalytic reactivity of Cu 2O particles decorated with Co 3O 4 nanoparticles of different sizes and shapes in terms of amounts and rates of hydrogen evolved by AB hydrolysis indicated that the size of Co 3O 4 nanoparticles is more important than the shape to exhibit high catalytic reactivity.