Small carbon nano-onions (S-CNOs) were prepared by annealing nanodiamonds (ND) in an argon atmosphere. The structure and morphology of S-CNOs were determined by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) and the average grain size of the S-CNOs was about 8 nm. In situ high pressure Raman spectra of S-CNOs were investigated by diamond anvil cell experiments at pressures up to 22.5 GPa. A reversible structural transition occurred at about 7.4 GPa, resulting from the polygonization of S-CNOs. The structural transition pressure of S-CNOs is lower than that of large CNOs (L-CNOs) and onion-like carbon (OLC) nanospheres. S-CNOs derived from the annealing of ND have a high defect density, a large number of sp³ bonds and high free energy. In addition, high pressure can be generated in the interior of S-CNOs at high temperatures. The results indicated that nanotwinned diamond (nt-diamond) may be prepared using S-CNOs derived from the annealing of NDs as a raw material below 10 GPa, which is much lower than the pressure needed for synthesizing nano-polycrystalline diamond (NPD) and nt-diamond with other carbon resources (usually more than 15 GPa).