The purpose of this research was to investigate the effect of the main factors of the surface roughness in aluminum semi-solid 2024 face milling. The results of the research could be applied in the manufacture of automotive components and mold industries. This study was conducted by using computer numerical controlled milling machine with 63 millimeter diameters fine type carbide tool with twin cutting edge. The controlled factors were the speed, the feed rate and the depth of cut which the depth of cut was not over 1mm. For this experiment, we used factorial designs and the result showed that the factors effected of surface roughness was the feed rate and the speed while the depth of cut did not effect with the surface roughness. Furthermore, the surface roughness was likely to reduce when the speed was 3,600rpm and the feed rates was 1,000mm/min. The result of the research led to the linear equation measurement value which was Ra = 0.205 - 0.000022 Speed + 0.000031 Feed rate. The equation formula should be used with the speed in the range of 2,400 - 3,600rpm, feed rate in the range of 1,000 - 1,500mm/min and the depth of cut not over 1mm. The equation was used to confirm the research results, it was found that the mean absolute percentage error (MAPE) of the surface roughness obtained from the predictive comparing to the value of the experiment was 3.48%, which was less than the specified error and it was acceptable.