In new designed workpieces, surface roughness is a very important parameter which affects lifetime and working quality of the parts. Nowadays, manufacturing of parts with very low surface roughness is a key purpose for most of advanced industries. Sometimes it is impossible or very hard to achieve fine surface finish by conventional processes. Surface finish, accuracy and integrity of parts produced by conventional machining methods depends on machine tool system including machine tool, cutting tool, workpiece, machining conditions and cutting fluid. To achieve high surface roughness, machine tool should be rigid, free from static (form deviation) and dynamic (chatter or vibrations) errors, and also tool and workpiece errors. Furthermore, with development of new hard to finish materials such as superalloys and ceramics and composite materials, variation of forces in machining process is natural and can not be avoided by changing of machine tool system. Also, materials such as ceramics and glasses are inherently brittle, and failure of parts made of these materials is usually due to cracks created during machining process. Magnetic abrasive finishing (MAF) is one of the modern finishing methods which can be used to produce nanometer range roughness on flat surfaces and also on inner and outer surfaces of cylindrical parts. This method can also be used for some other geometries. This process is suitable for working on hard materials such as ceramics and superalloys which are hard to be finished with conventional methods. Bevel gears are included in a group of mechanical parts which are used widely in various industries. Surface roughness of gear tooth is one of the important parameters affecting the lifetime of gear and its noise during working process. In this study, by adding of a rotary axis to a 3-axis miniature CNC mill, a system is designed and manufactured for finishing of spiral bevel gears. This system can be used to study and investigate the effects of some important factors of MAF process. This system also can be used for finishing of other types of gears (straight bevel gears, helical gears, spur gears) and also outer surface of shafts. Motion control of the axes of this system is done using a software called "MACH3", and this reduces the costs of system considerably. After preparing the system, design of experiments (response surface method) is used to design appropriate experiments for studying of factors effects on ultimate surface roughness. Then, statistical analysis is done by MINITAB software on experimental data and important factors and their interactions are determined, and finally a regression equation for estimation of surface roughness from process variables is obtained. Analysis results showed that all of the studied factors, i.e., rotational speed of magnetic tool, current of electric coil, and mesh number of abrasive particles have a significant effect on ultimate surface roughness. Among the studied factors, current has maximum effect and rotational speed of the tool has minimum effect on reduction of surface roughness. Experiments showed that finishing of bevel gears by magnetic abrasive finishing process needs a long time, so finishing of gears by this method is not efficient in usual cases, however it can be used in particular cases which require high surface finish. Keywords: magnetic abrasive finishing, surface roughness, abrasive powder, magnetic field, design of experiments, analysis of variance