This paper researches on the swept volume in the numerical control (NC) simulation system and proposes the new method of swept volume generation in three-axis machining and in five-axis machining. Based on general tool model and swept volume theory, swept volume generation methods are presented for different tool trajectory and motion mode. The test has proved that the proposed methods are feasible and effective. Material removal process in NC machining simulation can be accurately displayed with fast computation and less memory consumption.