In this research the Magnetic Abrasive Finishing capabilities in AISI 4140 steel polishing was studied. Surface roughness was considered as a function of the tool rotational speed, working gap, abrasive material and abrasives particle mesh size. To conduct the tests, a specific magnetic tool was designed to polish the flat steel surfaces by a milling machine. Experiments were arranged based on Taguchi method and using abrasive material consisting of Aluminum Oxide or Silicon Carbide with paraffin oil and Carbonyl Iron powders with different mesh sizes. After surface roughness measurement of samples, the effect of each parameter on the surface quality was inspected by ANOVA method. Results showed that in Magnetic Abrasive Finishing of mentioned steel, the parameters of working gap, tool rotational speed and the abrasive material type are of the most importance as ordered. Finally, a roughness predictor function was introduced by regression method.