B-C-N films have been synthesized by ion-beam-assisted deposition technique, in which boron and carbon were evaporated by electron beam and a mixed nitrogen and argon ion beam was simultaneously irradiated onto silicon (100) substrates. The ratio of argon ions to nitrogen ions was varied by the flow rate ratio of Ar and N₂ gases fed into the ion source. In this experiment, the influence of the ratio on the mechanical properties and the microstructure were investigated. Nano-indentation studies showed the maximum hardness up to 23 GPa at the gas ratio of 25-50%. The films prepared under the appropriate conditions indicated low friction coefficient of 0.04-0.08 against a sapphire pin and excellent wear resistance. The existence of cubic-phase B-C-N crystal-line in the film which has been predicted as one of new hard materials was revealed by transmission electron microscopy image. It was concluded that the superior mechanical properties prepared at the optimum gas ratio could be attributed to the appearance of the new cubic phase.