In order to meet requirements of transforming the ladder diagram programmable logic controller (PLC) into an instruction table for numerical control (NC) system in machine tool, the PLC ladder diagram was divided into some structural components including base-line, ring, semi-ring and ring-base-line, and etc. By using these components, the validity of PLC ladder diagram was checked, the relationship between PLC ladder diagram and instruction table was analyzed, and the algorithm about transformation from PLC ladder diagram to instruction table was designed. The application examples show that the algorithm is efficient and has good real time, which is suitable for the NC system of machine tool.