This paper presents a prediction model of flank milling forces in ruled surface impeller machining process with cylindrical, helical, end mills. Flank milling is widely used in machining objects such as gas turbines, impellers, fan vanes and all workpieces defined by non-developable, ruled surfaces. In the present paper, a short review on the development of milling force prediction is given first, and then a simple introduction about ruled surfaces is presented. Having defined the milling surfaces, flank milling forces with cylindrical milling cutters are analyzed. A cutting force prediction model of tangential, radial and axial is evaluated. The methodology allows prediction of average cutting load on the tool and workpiece, which can be used for cutting tool selection and chatter vibration avoidance. At last, the proposed model is demonstrated experimentally in milling an impeller with ruled surfaces.