Many die/mold parts including sculptured surfaces are usually completed by pocketting with ball end mill. Therefore the precision machining in pocketting is important. It has been noted that the tool deflection produced by the cutting force is a main cause of machining error in pocketting. The instantaneous deflection is dependent upon the static stiffness of tool and the instantaneous cutting force. In this study, the stiffness of holder was obtained by experiment and the stiffness of cutter was analyzed by the discrete elements along cutter length. The instantaneous cutting force was obtained by using the iteration method with the linear interpolation of basic machining data. By predicting the tool deflection in pocketting, a method improving the machining accuracy is suggested.