Abstract: This paper introduces briefly an expert system on numerical control automatic programing,including knowledge representation,blackboard architecture with different levels,problem solving strategy,forward reasoning and interpreter.A set of knowledge sources is adopted to deal with different combination of curves.Because of using coding technique on knowledge processing,heuristic search and pruning technique,the time and space efficiency is highly improved.The planning program of the system can produce cutting path automatically,give NC code output executed by NC lathe,display graph of parts and simulate cutting action.At present,the prototype system written by FORTRAN has been running on IBM/PC and SUN3/60 computer.