STEP-NC is a new CNC data interface proposed to replace ISO 6983, and has been partly formed into the draft international standard (ISO 14649). In this paper, the STEP-NC-based part program is analyzed, and the general methodology and tools for STEP-compliant application development are introduced. Then a framework for STEP-NC interpreter is proposed based on the SDAI method and the implementation solutions are presented. The EXPRESS compiler is used to convert the milling schema into C++ classes, and the information is extracted from the file in aid of data access tools. The said interpreter, which is fundamental to STEP-NC based machining, is also implemented with supporting tools in the ST-Developer environments. It is used to read the STEP-NC file into ROSE working form, and then extract and organize the manufacturing information in an object-oriented way.