Ultra-small　Pd　nanoparticles　were　successfully　prepared　through　reduction　of　a　supramolecular　assembly　formed　between　the　macrocyclic　decamethylcucurbit[5]uril　(Me10CB[5])　and　[PdCl4](2-)　anions　via　H-bonds.　The　final　Pd　NPs　exhibit　excellent　activity　and　stability　toward　the　electrochemical　reduction　of　CO2　to　CO.　This　work　provides　a　new　strategy　for　the　preparation　of　nanocatalysts.