Introduction Transparent ceramics are polycrystalline inorganic nonmetallic materials that are transparent in the visible range and have some superior characteristics (i.e., high melting point, high strength, high insulation, corrosion resistance, high temperature resistance, and good light transmission). Diamond as a hardest substance in nature has an extremely high thermal conductivity, a wide spectral transmission range and a good chemical stability as an ideal transparent ceramic material in harsh and extreme environments. However, the single crystal of diamond is brittle and easily broken along the cleavage plane (111), greatly restricting the application of single crystal diamond. Nanocrystalline diamond (NPD) is a polymer of small diamond particles in nano-scale, and the grains are directly bonded by diamond to form a compact structure. The macroscopic properties of nanocrystalline diamond show an isotropy, and there is no directional and disfoliating plane due to the disordered accumulation between particles, having the better mechanical properties, compared to single crystal diamond. The nano-polycrystalline diamond can be prepared, but the preparation of transparent nano-polycrystalline diamond and its application in the field of light transmission are rarely reported. Little work on the transparent mechanism and defect formation mechanism of nano-polycrystalline diamond have been done yet. In this paper, a nanocrystalline diamond was prepared by a high-temperature and high-pressure method. In addition, the mechanical properties were also investigated.Methods A high purity graphite powder (99.999 9% in mass fraction) as a precursor was prepressed into a cylindrical shape ( = 2.5 mm, h = 2 mm), wrapped with aluminum oxide, and assembled. The pressure was uniformly increased to 15 GPa for 8?h. Afterwards, a rhenium (Re) heater was used for heating, and the temperature was uniformly increased to 1 800, 2 300 ℃, and 2?600 ℃ at 200 ℃/min. After holding for while, the temperature was slowly reduced to room temperature at 200 ℃/min, and the pressure was slowly relieved to normal pressure after 15 h. The polished sample was determined by an optical microscope. The phase composition of the sample was analyzed by a model Rigaku FR-X X-ray diffractometer (target Mo, wavelength λ = 0.709 3 ?). The Raman spectra were determined by a model Mono Vista CRS+ Raman spectrometer at an excitation wavelength of 532 nm. The surface morphology of the samples was analyzed by a model Hitachi SU-70 scanning electron microscope. The transmission samples were prepared by focusing ion beam (FIB), and the electron diffraction (SAED) and high-resolution transmission electron microscopic images of the samples were characterized by a model JEM-2200FS transmission electron microscope (HRTEM).Results and discussion A high-temperature and high-pressure method is an effective way to prepare high performance transparent ceramics. Graphite as a raw material can be transformed into a transparent nano-polycrystalline diamond at 15 GPa and 2 600 ℃ for 1 min. Also, the temperature gradient of the sample cavity leads to a non-uniform transparency of the sample, and the increase of temperature is conducive to the increase of the transparency of the nanocrystalline diamond. The results of morphology analysis show that the sample has a layer structure of martensitic phase transformation and a uniform particle structure of diffusion phase transformation. The layer thickness and grain size are approximately 100 nm. High pressure and stress cause a partial dislocation within the sample, improving the mechanical strength of the sample. Nano-polycrystalline diamond as the maximum hardness transparent ceramic is expected to be widely used in the field of special optical windows under extreme conditions.Conclusions Graphite powder was selected as a precursor material and pre-pressed into a 2.5 mm diameter and 2 mm high cylinder. The Kawai 6-8 press device (1 000 t) was used to synthesize a nano-polycrystalline diamond at 15 GPa and 1 800-2 600 ℃. The results showed that the partially transparent nano-polycrystalline diamond could be synthesized under the conversion boundary conditions of synthetic transparent diamond, and the complete transformation from graphite to diamond was completed at 15 GPa and 2 600 ℃, and the completely transparent nano-polycrystalline diamond was synthesized. The results by XRD and SEM analysis indicated that the prepared samples were a pure cubic phase, there was no preferred orientation, and the grain size was approximately 100 nm. Nano-polycrystalline diamond as the hardest transparent ceramic could be used to manufacture high-performance transparent ceramics, which have a promising application in military, industrial and other fields.