Rotary ultrasonic machining process, regarded as one of the effective processing methods for hard-brittle materials, is introduced into drilling K9 glass in this paper. The effective cutting velocity, cutting depth, and cutting length of single diamond particle are determined by analyzing the kinematics characteristic of diamond tool in rotary ultrasonic drilling (RUD). Experiments are conducted to study the influences of process variables (spindle speed, feedrate) on cutting force, chipping size, and surface roughness in RUD. As comparison study, the processing performances between RUD and diamond drilling are also discussed. The experimental results show that the RUD process can significantly reduce cutting force and the value of chippings size, which inferred that RUD process can improve machining efficiency and make the machining cost lower. It is also concluded that the effective cutting depth of diamond particles is the main factor for surface roughness in RUD of K9 glass.