Abstract: Three-dimensional (3D) GD&T provides basic information for tolerance visualization and generation of assembly dimension chains. This paper presents an approach to 3D GD&T for mechanical products in virtual assembly system. Points for dimension lines and control frames are derived from ACIS-based geometry, and a 3D GD&T model referring to the international standard is developed. In addition, 3D GD&T's consequent transformation due to part movement is introduced, and an assistant angular dimension model is built for 3D linear dimensioning. Finally, a prototype system is presented.