We report the high stability and reversibility of disordered graphitic nanocarbon (DGN) under high pressure. No bonding change can be observed in DGN even up to 52.0 GPa and it fully recovers upon decompression. The high stability and reversibility should be related to its unique highly disordered structure which consists of nanosized wavy graphene layers, making the DGN elastic upon compression. Combined with high temperature, DGN is transformed into polycrystalline nanodiamonds with average grain size smaller than the pristine nanoparticles.