A method for manufacturing cubic diamond nanocrystals (10) comprising the following successive steps: (a) providing crystalline diamond powder where the maximum particle size of the powder is equal or more than 2 um and equal or less than 1 mm; (b) milling said crystalline micron diamond powder using nitrogen jet milling micronization so as to manufacture a fine powder; (c) nanomilling the fine powder of step b) using a planetary tungsten carbide ball mill; (d) acid treating the nanomilled powder of step c); (e) extracting the cubic diamond nanocrystals (10) by centrifugation. Advantageously round-shaped cubic diamond nanocrystals are manufactured.