In this paper, a virtual machining system (VMS) for the ultra-precision diamond turning is presented, and the NC tool path translator function module of the VMS is discussed. The developed VMS is based on the NC program not as some commercial CAD/CAM software which is based on the cutter location DATA file. In the VMS, the virtual machine tool is controlled through the NC program used by the actual machine tool. The NC tool path translator consists of accidence analysis, lexical-analyzer, grammar and semantic parse, driving code generator, etc. The manufacture of a mould insert for a diffractive lens was used as a case study to illustrate the performance of the developed VMS.