Serials of ultrasonic face machining experiments were designed and carried out on glass materials with free abrasives. Two different kinds of ultrasonic machining, namely, non-rotary ultrasonic machining (NRUSM) with free abrasives and rotary ultrasonic machining (RUSM) with free abrasives were compared. Two systems were set up to in-process monitor the changes of static forces and frequency respectively. The effects of static force, spindle speed and amplitude of ultrasonic vibration on material removal rate (MRR) and surface roughness were analyzed. The surface of workpieces was also observed by a digital video microscope system. The experimental results indicated that, at the given conditions, there exists an optimal value of static force and amplitude of vibration to obtain the maximum MRR, and RUSM was found to be superior to NRUSM in the MRR, but inversely in surface roughness.