The effects of processing parameters (amplitude, milling speed, feed per tooth) on surface quality machined by side blades in ultrasonic torsional vibration milling (UTVM) were investigated. Based on a series of UTVM experiments, the influence of single-variable processing parameter on surface quality of milled surfaces was obtained. Furthermore, the significance of these effects of each factor and interactive factors of processing parameters on surface roughness was studied by variance analysis and response surface methodology. Processing parameters were optimized and prediction model of surface roughness was established. The experimental results show that applied ultrasonic torsional vibration can obviously reduce surface roughness of milled surfaces, and the amplitude is the most significant factor of affecting surface roughness in UTVM, while the milling speed and feed per tooth are the second and the third, respecitively. Large amplitude and low milling speed are more likely to improve the roughness of the milled surfaces.