Cu₂O particles are active catalysts for hydrogen evolution from ammonia borane (AB) by hydrolysis, however, Cu₂O particles easily form agglomerates as a result of highly reduced conditions during the reaction. In order to suppress agglomerate formation, capping of Cu₂O with organic reagents or inorganic materials was performed and the catalytic reactivity in AB hydrolysis was examined. Among the examined methods, capping of Cu₂O particles with Co₃O₄ nanoparticles was the most effective to avoid agglomerate formation of Cu₂O particles. The finding enabled us to examine the shape effect of Cu₂O particles on the catalytic reactivity in AB hydrolysis in the presence of Co₃O₄ nanoparticles. Comparisons of turnover frequencies for hydrogen evolution of Cu₂O–Co₃O₄ composites, in which Cu₂O particles were in the shape of 50-facets, cube, octahedron or rhombicuboctahedron, indicated that the composite with Cu₂O with the shape of 50-facets showed more than 7-fold higher hydrogen evolution rate normalized by surface area than the composite with Cu₂O with the octahedral shape. The size and shape effects of Co₃O₄ nanoparticles were also investigated on their ability to protect Cu₂O from agglomeration. Comparisons of the catalytic reactivity of Cu₂O particles decorated with Co₃O₄ nanoparticles of different sizes and shapes in terms of amounts and rates of hydrogen evolved by AB hydrolysis indicated that the size of Co₃O₄ nanoparticles is more important than the shape to exhibit high catalytic reactivity.