K9 glass is regarded as one of the most difficult-to-machine materials due to its mechanical characteristics, which is known as an excellent performance optical material. In this paper, rotary ultrasonic face milling (RUFM) process is introduced into machining flat surface of K9 glass using right-angle diamond cutter for the first time. It studies the influence of process variables (spindle speed, feedrate, cutting depth, and cutting width) on cutting force and surface roughness through the single-factor experiments. The cutting force is measured by a KISTLER dynamometer, and the surface roughness is measured by a talysurf. The investigation also includes a comparison between RUFM and diamond milling. The experimental results tell that the RUFM process can significantly reduce cutting force, which inferred that RUFM can have less tool wear and longer tool life. It also shows that the surface roughness in RUFM of K9 glass is slightly higher than that obtained in diamond milling.