An intelligent STEP-compliant CNC controller was studied and designed under Linux Rtai. A CNC expert knowledge-base was established by importing the theory of ontology. The hardware interface in the bottom layer was realized beyond the self-developed real-time Ethernet fieldbus platform. A prototype was developed under universal Linux with Rtai embedded and the feasibility of this scenario was validated. This CNC controller had the capability of programming and re-planning on the shop floor. It could make decisions by itself to some extent and support distributed control. The whole system was characterized by openness, intelligence and flexibility.