In the book of Huazhong NC system as an example, introduces the background and development of CNC machine tools, CNC machining characteristics and scope of application, the composition of CNC machine tools, working principle and function. On the basic concept of NC programming, NC programming, the contents and methods of the machine coordinate system, the determination of the workpiece coordinate system in NC machine tool, the origin of the coordinate system and the reference point of NC programming, program structure and format are also introduced.