Generally, cutting force models use a sin function method to calculate chip thickness. In slot end milling, the error from a sin function method is much bigger than other machining because a tool rotation angle in cutting is much larger. Thus in this paper, a new method to calculate chip thickness was suggested and evaluated. In a new method, tool position data according to tool rotation are checked and stored so that it is possible correct chip thickness is calculated. Cutting force waveforms simulated from a sin function method and a new method and measured waveforms from experiments were compared and error percentages were obtained. Finally, a new method had good results for simulating cutting force in slot end milling.