A new automatic operation planning system for NC lathes is developed. Contour of a workpiece and several possible clamping positions are the input data. The domain between contours of a blank-part and a finished-part is divided into subdomains called blocks. The system assigns to each block several single operations or complex operations by which the block can be removed completely with no collisions between workpiece and tools or turrets. A single operation denotes one tool having a setting angle and a feed direction. A complex operation denotes a permutation of single operations which must be applied successively to one block. The correspondence between blocks and operations is transformed into conjunctive normal form in relation to single operations in order to determine a set of a minimum of single operations for the workpiece. NC tape and the indication of a minimum setting length of each tool from turret are generated together with an operation plan. By editing the adaptability values of operations for block patterns, the knowledge and the experience of users can be reflected to the system.