Rotary ultrasonic machining (RUM) has been used to machine both brittle and ductile materials as well as composite materials. There are numerous reported studies about the effects of various process variables on output responses. However, the current literature contains few articles about the measurement methods of vibration amplitude in RUM and about the effects of process variables on vibration amplitude. The lack of such knowledge has made it difficult to explain some experimentally observed phenomena in RUM and degraded the creditability of some experimental results with RUM. This paper, for the first time, presents a measurement method capable of measuring vibration amplitude during RUM machining. It also reports RUM experimental results on effects of cutting tool, ultrasonic power, workpiece material, tool rotation speed, and feedrate on ultrasonic amplitude. This study will fill some blanks in the literature and provide plausible explanations to some seemingly contradictory results reported in the literature.