A new methodology to automate machining operation planning is proposed. A new machining operation plan is reconfigured from past case data or past machining operations. Machining information, e.g. cutting tool, cutting conditions, tool path pattern, is associated with a machining feature. A machining feature to be cut is related to 3D CAD model of workpiece. Sets of machining information and a 3D CAD model is stored as the past case data. In order to generate a new process plan of a new workpiece, machining features are recognized first. Then, each of machining feature is compared with machining features contained in 3D CAD models of workpieces stored as the past case data, and the most similar machining feature is selected. The machining information associated with the selected machining feature can be applied for each of machining feature consisted in the new workpiece.