Rotary ultrasonic face milling (RUFM) process is introduced into flat surface machining of K9 glass in this study. The model of material removal for RUFM is developed. The cutting force, tool wear and subsurface damage of RUFM and diamond milling of K9 glass are compared. The results present that the relationship between cutting depth and ultrasonic amplitude has remarkable effects on cutting force, and suggest that cutting depth should be smaller than ultrasonic amplitude with RUFM process to obtain better processing performance. The experiments conducted as cutting depth is smaller than ultrasonic amplitude show that RUFM process can significantly reduce the cutting force, tool wear on the end face, and subsurface damage depth. Brittle deformation, materials removal and sub-surface damage features of K9 glass in RUFM and diamond milling are observed by scanning electron microscopy. The study indicates that the established material removal model for RUFM is reliable.