In this report, a novel and efficient purification technique for multi-walled carbon nanotubes (MWNTs) combining wet grinding, hydrothermal treatment, and oxidation processes has been described. Graphite pieces of over few tens of mum were removed by wet grinding in water followed by sieving. The separation of graphite pieces of the order of a few mum and MWNTs was achieved through hydrothermal treatment followed by centrifuging. Finally, the carbon nanoparticles were removed by burning the semi-purified sample in an oxidizing atmosphere at 973 K. Scanning electron microscope, transmission electron microscope, and Raman spectroscopy were used to monitor the purification efficiency at each processing step. Using this purification method, 16 mg of the purified MWNTs were obtained from 850 mg of the inner core material from the negative electrode.