Abstract: Diamonds with two extreme sizes, large single crystal and nanocrystalline, have completely different properties, and have aroused the continuous attention of researchers. Each has its own merits and can be converted into the other. The synthesis of large single crystal diamond can be described as the aggregation, assembly and combination of nanocrystalline nuclei, i.e., diamond transforming from the nano-scale to the inch scale. A large single crystal diamond can be transformed into nanocrystals by surface nanocrystallization. The preparation methods, properties and applications of single crystal diamonds of different sizes are introduced and the transformations between them are described. Research interest in controlling the crystal size is discussed.