Abstract: The size-dependent Kirkendall effect is studied by using Co nanoparticles. The sizes of Co nanoparticles differing by their crystal structures called nanocrystallinity, namely amorphous, polycrystalline fcc, single crystalline hcp, and single crystalline ε phase, are modulated from 4 to 10 nm. The nanoparticles self-assembled in 2D superlattices and differing by their nanocrystallinities are subjected to oxygen at 200 °C for 10 min.c...