Thin films containing boron have been prepared on silicon substrates by DC magnetron sputtering using a sintered boron carbide (B4C) target. The influences of N₂ or H₂ addition as a reactive gas on chemical bonding states, nano-indentation hardness, adhesive strength to the substrate and the friction coefficient of the films are investigated. Nano-indentation studies show that N₂ addition in the film formation process leads to a lower hardness of the film. Absorption spectra in Fourier-transform infrared spectroscopy exhibit peaks corresponding to hexagonal boron nitride, and C 1s peaks and N 1s peak in X-ray photoelectron spectroscopy spectra indicate that nitrogen dose not bond to carbon. The lack of a three-dimensional boron-carbon-nitrogen network in the films is thought to be a reason for the deterioration in the nano-indentation hardness of the films. On the other hand, the adhesive strength is increased significantly by N₂ or H₂ addition in the film formation process, although the frictional properties of the film against a steel ball in an ambient atmosphere are not improved in this study.