The expert system based on Case-Based Reasoning (CBR) to select cutting condition has been developed. In CBR, a new problem is solved by a similar past case. The developed system has the case database consisting of past cases or cutting conditions of high speed milling operation performed. The case database is equivalent to the experience and the practical background of skilled engineers in the system. At first, the system retrieves similar past milling operations to the required operation using several rules which define the similarity between tool shape, work material and so on. Then, the cutting conditions in the retrieved cases are adapted to the required operation. Finally, the suitable cases among adapted one are selected to determine the cutting conditions to the required operation. The cutting conditions suggested by the system are reasonable to the various end milling operations.