Abstract: Down milling processing test was conducted on carbon fiber reinforced plastics (CFRP) of four fiber directions (0°, 45°, 90°, 135°) by using a diamond coated cemented carbide tool. The effects of fiber direction and feed per rotation on processing surface quality were analyzed through comparing milling force and processing surface morphology. The results show that the main cutting force increases with the increase of feed per rotation. The maximal main cutting force is obtained with 0° fiber direction, while the main cutting force reaches its minimum value with 90° fiber direction. With the increase of feed per rotation, more broken resin and fiber could be left on surface of the workpiece with 0° fiber direction, while the adhesion of soften resin could be lightened on surface of the workpiece with 135° fiber direction. There are many shallow voids on surface of workpiece with 90° fiber direction, delamination of top and bottom laminate layers are observed at the same time. The finished surface of the workpiece with 45° fiber direction shows groove or wave-like surface morphology.