Abstract: A blind watermarking algorithm for 2D engineering graphics based on entity substitution and mean value of wavelet coefficients is proposed. Firstly, each line entity of the 2D engineering graphic is substituted by a polyline with k+1 vertices, and the substitution will not change the appearance of the 2D engineering graphic. Then the coordinates of the vertices in polylines are acquired to make some transformations to obtain the invariant abilities such as translation, scaling and rotation. After that, the coordinates of the transformed vertices are constructed into different transformation units. Finally, the discrete wavelet transformation is done to every transformation unit, and watermark is embedded to the mean value of wavelet coefficients of each transformation unit. Meanwhile, chaotic encryption and chaotic decryption are also used in the watermark embedding and watermark extraction, respectively, to improve the security of the watermark algorithm. Experimental results and analysis show that the proposed algorithm is blind detection, and it has a good capability of imperceptibility and has a high efficiency. At the same time, it is robust to the attacks such as rotation, translation, even scaling and graphic addition. It may be a potential new method for the copyright protection for the 2D CAD engineering graphics.