The aim of this research was to study factors, which was influenced on surface roughness in vertical milling of hardened medium carbon steel. The specimen was medium carbon steel grade JIS S50C that was hardened at 56± 2 HRC. Full factorial experimental design was conducted by 3 factors and 3 levels (33 design) with 2 replications. Studied factors were consisted of cutting speed, feed rate, and air coolant pressure. The results revealed that influenced factor affected to surface roughness was cutting speed and feed rate which showed significantly different. Higher cutting speed would cause on better surface quality. On the other hand, poorer surface quality was produced by higher feed rate. However, factors interaction were found between cutting speed with air coolant pressure and feed rate with air coolant pressure that significantly influenced to surface roughness. The interaction of high cutting speed with high air coolant pressure would be better quality of surface finish and lower feed rate with high air coolant pressure would be better surface quality.