Due to the high complexity and the low speed of the wire-EDM process, it is important to recognize all the possible errors before the working out process. The unproductive and time consuming test runs of NC-programs in the real production environment can be avoided by the graphic simulation. In this paper, a data structure and an algorithm are introduced to verify the NC code through the aid of graphics. Non-parametric R-map model is a special form of discrete surface representation in which the r-values of the surface are given at grid points. R-map is used to compare the final workpiece-geometry with the CAD data. The verification and editing system for wire-EDM NC-programs is implemented. The developed software displays the trace of the wire electrode and the end-product. The aim of the verification and editing is to recognize and eliminate errors and weak points before beginning actual production.