This paper studies digital watermark technology of numeric attributes in relational database for data's copyright protection. It analyzes current researches on this issue and proposes a new watermark mechanism (NWM) using classifying and twice majority voting method, which pays much attention to data's usability through relational structure and semantic constraints, marks each attribute separately, and makes marked bits unobtrusive and robust to various attacks. We implement the watermarking algorithm and give the experimental evaluation.