A method for obtaining high purity carbon nano-onions starting from nano-diamonds, comprising the steps of heating the nano- diamonds up to a first predetermined temperature of 1,200- 1,800°C in a protective helium atmosphere; maintaining said first temperature constant for a first predetermined time; and slowly cooling the carbon nano-onions; wherein the heating step i) is performed by means of at least two successive heating ramps, a first ramp in which the nano-diamonds are brought to a second predetermined temperature, greater than 1000°C, and are maintained at said second temperature for a second predetermined time, and a second ramp in which the mixture of nano-diamonds in the transformation step into carbon nano-onions is brought to the first temperature, then maintaining the first temperature constant for the first predetermined time.