Nanodiamond films synthesized by a hot-filament chemical-vapor-deposition method using a methane-hydrogen mixture have been investigated with near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The carbon K-edge NEXAFS of the nanodiamond film exhibits a blue-shifted exciton peak and absorption threshold relative to that of the CVD diamond film. These features are characteristic of quantum confinement behavior. This observation confirms the presence of a nanodiamond film as was revealed by transmission electron spectroscopy. The results show that the correlation of the blue shift with the grain size of the nanodiamond is in the same direction but smaller than results previously reported in the literature. (C) 2003 Elsevier Science B.V. All rights reserved.