To protect against unlawful reproductions and distribution, the current paper proposes a digital watermarking technique for architectural drawings produced using a CAD system. First, the POLYLINEs are extracted from the drawing; then, an adaptive algorithm is used to embed a watermark in the characteristics of each POLYLINE. Next, the CIRCLEs are embedded using an adaptive watermarking algorithm related to the radius of circle from drawing. The proposed watermarking scheme is robust to various attacks, such as the geometrical transformation. Additionally, the proposed method satisfies the requirement of transparency for CAD program. It used AutoCAD 2002, which is commonly used as a CAD program for experiments. Experimental results confirmed the robustness and invisibility of the embedded watermarks in several conversions of an architectural drawing.