In this work, we developed X-ray radiation detectors with sandwich structure fabricated from nano-crystalline diamond (NCD) films. These NCD films with different grain size ranging from 15 nm to 160 nm were grown on silicon substrates using a hot-filament chemical vapor deposition technique. I-V measurement results indicate that with reducing of the grain size, the resistivity of diamond films decreases from 9.5x10(8) to 6.20x10(7) Omega cm and the ratio of the photocurrent to the dark-current (I(ph)/I(d)) of the detectors decreases rapidly from 0.45 to 0.09 at an electric field of 50 kV/cm. Typical spectral response to 5.9 keV (55)Fe X-rays shows that counting efficiency and energy resolution of NCD detectors with large grains are better than those of detectors with small grains, due to the less defects and grain-boundaries contained in the film. (C) 2011 Elsevier B.V. All rights reserved.