Abstract: Different from traditional one-step tool path calculation，a two-step approach is presented to improve the controllability and visibility of NC machining．A new term Numerical Control Feature Unit （NCFU） is introduced．At first the workpiece is subdivided into NCFU and then NC tool path is generated surrounding each NCFU ．Because the second step calculation is relatively simple，it can be finished by CNC controller in real time．In this paper，the abstract NCFU and Contour-Parallel machining Feature Unit （CPFU） for 2．5axis machining are defined，and an algorithm for processing CPFU is provided．