We investigated inherent electronic properties of ultra-hard nano-polycrystalline diamond (NPD) synthesised at 15 GPa and 2300 °C. NPD exhibited a p-type semiconducting property with a resistivity of 1 × 106 and 4 × 102 Ω cm at 400 °C and 800 °C, respectively, with activation energies as high as 1.2 eV. The mobility was 2 cm2/Vs at 800 °C. The observed transport was considered to be attributed to grain boundary. Cathodoluminescence spectra show defect-related peaks with dominant peaks at 2.8 eV close to band-A and peaks below 2 eV, which is possibly related to crystal defects. In contrast, photoluminescence excited with a wavelength of 405 nm resonantly excited peaks having energies lower than 2.1 eV. The large density of crystalline defects such as grain boundaries and impurities in the NPD would result in the characteristic electronic and optical properties.