In conventional machining of free form surfaces, incomplete information exchange between CNC and CAD/CAM results in many limitations need to be rectified. In the paper, a new structure of CNC based on STEP-NC is proposed. A 5-axis real-time interpolator for NURBS surfaces machining is realized. A STEP-NC controller has been developed, which is a software modular CNC system. The CNC system has been installed on a tilt-rotary type 5-axis machine tool. The milling experiment has been performed.