The optical properties of ultrathin (less than 100nm) boron-doped nanocrystalline diamond (B-NCD) film were investigated in a wavelength range of 200–20,000nm. The B-NCD refractive index showed values close to that of monocrystalline diamond (n=2.45) in a broad wavelength range (450–4000nm). A transmittance up to 70% and the average film thickness of 70nm were achieved. A special cone-shaped shim was used in the deposition process. Ultrathin nanocrystalline films were deposited on silicon substrates using the Microwave Plasma Assisted Chemical Vapour Deposition (MW PA CVD) method.Cyclic voltammetry (CV) measurements in aqueous media consisting of 5mMK 3 [Fe(CN) 6 ] in 0.1M Na 2 SO 4 demonstrated a width of the electrochemical window up to 2.5V. The evolution of the surface morphology was analysed using a scanning electron microscope (SEM) and an atomic force microscope (AFM). The chemical composition of B-NCD films was examined with micro-Raman Spectroscopy. The Raman spectra included a diamond peak and a nanocrystalline diamond band at 1330–1333cm −1 and 1126–1143cm −1 , respectively. The thickness and optical properties of ultrathin B-NCD film in UV–MIR wavelength range were investigated using spectroscopic ellipsometry.