Abstract: Although segmentation of a free-form surface is widely used in engineering field, little work has been done on generating the accurate boundary curves of surface patches. In this paper, a general method of iso-surface segmentation and a novel approach based on pointwise tracking for boundary curve generation are presented. Firstly, the initial boundary point is found through gravity center sampling based on the theory of equiarea triangulation of parametric space. Secondly, starting from the initial boundary point, a set of boundary points are generated along the boundary curves by pointwise tracking. Thirdly, the boundary curves are defined through spline interpolation of all these points. Finally, an algorithm of automatic iso-surface segmentation for NC machining is established and examples are given to illustrate the validity and effectiveness of the approach.