An approach of cutting force prediction in circular milling for flat end mill is presented in this paper. To this end， the mathematical model of tool and workpiece intersection is established. By predicting the time varying cutter -part intersection as the cutter travels along the circular path，the changing entry and exit angles are analyzed. Meanwhile，the curvature effects of tool path on instantaneous chip thickness is analyzed. The cutting force coefficients are identified by slot milling. Based on the idea of differential，the acquired instantaneous chip thickness is utilized to calculate the cutting force of every unit according to the cutting force model. And then，the cutting force is achieved by means of integration. The results of simulation and milling experiment show that the predicted cutting force is according with the measurement both in magnitude and in variation trend. Thus，the validity of the proposed approach is confirmed.