Abstract: A new nanocrystalline diamond film is prepared in C₂H₆O/H₂ using self-made microwave plasma chemical vapor deposition(MPCVD)system.The nanocrystalline diamond film is deposited on the surface of(100)-oriented Si wafer.Then a nanocrystalline diamond vacuum window is achieved after the silicon wafer is corroded by a mixture of nitric acid and hydrofluoric acid.The morphology,grain size,microstructure,orientation or texture,and crystalline quality of the diamond samples are characterized by scanning electron microscopy(SEM),X-ray diffraction,micro-Raman spectroscopy,and atomic force microscopy(AFM).The final window has a thickness of 15 μm,with surface roughness of R_a 39.5 nm and grain size of 30 nm.According to self-made leakage rate system,the air leakage rate of this vacuum window is 8.8×10^-9 Pa·m³/s.