Abstract: Disclosed is a method for separating nanodiamond clusters synthesized by a detonation method having a size of 100 nm˜1,000 nm into nanodiamonds of 100 nm or less—more specifically, into uniformly sized nanodiamonds in the range of 5 nm˜50 nm, free of metal and alkaline impurities and ready to quantitatively attach functional groups on the surface of the nanodiamonds for applications such as thin film precursor materials, drug delivery systems and cosmetics compositions.