Abstract: To explore the influence factors and influence rules of the machined surface roughness of Si3N4 ceramic in the ultrasonic vibration milling-grinding process, the abrasive particle trajectories of the ultrasonic vibration grinding wheel were analyzed, and on this basis a mathematical model for the machined surface roughness was established. Using this model, the surface roughness of Si3N4 ceramic machined by the ultrasonic vibration milling-grinding was predicted and verified by experiments. The results show that the influences of the factors on the machined surface roughness from big to small were in the order of spindle speed, feeding rate, grinding depth and amplitude. The surface roughness decreased rapidly with the increase of the spindle speed, increased with the increase of the grinding depth and the feed speed, and had a downward trend with the increase of the amplitude. When Si3N4 ceramic was removed mainly by the ductile mode, the predicted machined surface roughness coincided well with the experimental results.