The development of dendrimer reactors opened the new research field by the production of metal subnanoparticles with definite atomicities. These precise platinum subnanoparticles exhibited the true catalytic properties, which have been hidden by the conventional synthetic method due to the substantial size distribution. One significant finding was that the several subnanoparticles exhibited much higher oxygen reduction reaction catalytic activity than the conventional platinum nanoparticle (∼3 nm). The result was completely opposite to the common notice that the most catalytically active particle size is ca. 3 nm. Despite of this inconsistence, it was finally concluded that some subnanoparticles have specific surfaces which have higher activities originated from the unique geometric structures.