Boron carbide thin films were deposited on silicon substrates using ion beam sputtering at different substrate temperatures. The chemical composition and the microstructure of the films were analyzed by X-ray photoelectron spectroscopy, and transmission electron microscopy, respectively. Although all films are amorphous despite their deposition temperature, their radial distribution functions derived from the transmission electron diffraction pattern disclose differences in the short-range order for films deposited below and above 350 degrees C. The nanoindentation hardness of the films was measured using a nanoindentor. The internal stress of the films was evaluated using a profiler based on Stoney's equation. The experimental results suggest that the different hardness of the films is determined by the microstructure, rather than external factors. (C) 2007 Elsevier B.V. All rights reserved.