High efficiency mirror surface machining of brittle materials such as glasses and ceramics has become more important as these materials have become used more widely in optical and electronic devices. In this work, the ultrasonic vibration diamond cutting of glasses was performed in order to investigate the effect of tool vibration on the brittle–ductile transition mechanism. The effect of cutting speed on the critical depth of cut was studied by groove cutting experiments. The value of critical depth of cut has been found to vary with the ratio of vibration speed to cutting speed. The characteristics of the surface generated in the ductile mode have been examined and the measures for minimizing the influence of tool vibration on surface finish are suggested.