In this study, a semi-fragile watermarking scheme based on vertex geometry is proposed for 3D model authentication. Watermark is generated from 3D model and random number generator by a seed number. To embed and extract watermark only needs the seed number and some simple arithmetic operations. Our method is robust to general 3-D model processes, such as uniform scaling, translation, and vertex reordering. The embedding process has little impact to the original 3D models that PSNR values can be up to eighties or nineties. Thus the proposed method is very suitable for precision instruments or medical science models. Experiments demonstrate that our method is time and performance satisfactory.