Ultrasonic-vibration-assisted grinding (UVAG) or rotary ultrasonic machining has been investigated both experimentally and theoretically. Effects of input variables on output variables in UVAG of brittle materials and titanium (Ti) have been studied experimentally. Models to predict the material removal rate in UVAG of brittle materials have been developed. However, there is no report on models of cutting force in UVAG. This paper presents a physics-based predictive model of cutting force in the UVAG of Ti. Using the model developed, influences of input variables on cutting force are predicted. These predicted influences are compared with those determined experimentally. This model can serve as a useful template and foundation for development of cutting force models in UVAG of other materials (such as ceramics and stainless steels) and models to predict torque, cutting temperature, tool wear, and surface roughness in UVAG.