Recently, as a great demand for high-strength and high-hardness materials is rising, improvement in performance of tools which manufacture those materials is being needed. Generally, tools to manufacture such materials are given high-hardness coating. Diamond and DLC (diamond like carbon) film are regarded as typical high-hardness materials, though, they can't be given to iron tools because carbons contained in them have high reactivity to iron. Consequently, the boron-nitride film which doesn't contain any carbons is expected as a coating material which can be adapted to iron tools. In this research, we prepared boron-nitride thin films on Si substrate using RF plasma CVD (chemical vapor deposition) method apparatus with a gas mixture of argon, nitrogen and trimethylborate (TMB: B(OCH₃)₃).