Complex surface mold has been widely used in various industries, and high efficiency and high quality can been achieved through high-speed CNC milling processing. Surface roughness including transverse and longitudinal roughness is an important criterion for mold quality. A high-speed milling experiment was performed in mold steel P20 using cemented carbide ball-end mill to investigate the surface roughness. The effects of process parameters on roughness including spindle speed, feed per tooth and radial cutting depth were examined, and an analysis on the mechanism for two kinds of roughness of different tool paths was finished. The experimental results show that the longitudinal roughness improve obviously while the spindle speed and the feed per tooth increase on the high-speed conditions, and the transverse roughness increase significantly when the radial cutting depth increases. And for a smaller roughness value, the tool path should be selected along the direction in which the curvature changes evidently.