Abstract: The dynamic contact process at the interface part between the stator and the rotor determines the output performance of a traveling wave rotary ultrasonic motor (TRUM). However, the elasticity of the flexible rotor is often neglected in the previous studies on the contact characteristic at the interface part. In this study, the effect of the flexible rotor elasticity on the dynamic contact behavior at the interface part was studied according to the dynamic coupling model by considering the elasticity of both the flexible rotor and the friction layer along three different directions. Both the dynamic contact behavior and the friction loss were studied from the perspectives of the spatial distribution of the contact interface, which included contact state, velocity, contact stress, friction stress, driving and breaking sub-zones. The results indicated that the flexible rotor exhibited the vibration velocity along the axial, radial, and tangential directions and formed a certain phase relationship with the stator along the circumferential direction, which affected the dynamic contact characteristic, frictional characteristic, and friction loss at the interface part. The phase consistency of the axial displacement distribution between the rotor and the stator indicated that the contact interface between the stator and the friction layer featured a gear meshing mechanism. The friction stress distribution in the contact zones revealed a novel mechanism, whereby the tangential friction stress drived at one end and brakes at the other end, which was very different from the previous distribution results of driving at the middle part and braking at both ends. The calculation results showed that both the elasticity of the rotor and the friction layer had a significant influence on the friction loss. The friction loss obtained by neglecting both the elasticity of the flexible rotor and the friction layer during the modeling was considerably larger than that by considering these two aspects. This phenomenon could be attributed to two reasons. Firstly, the radial vibration velocity from the opposite direction was the same with the direction of the stator radial velocity under the effect of the friction force, thus reducing the radial velocity difference and the radial friction loss between the friction layer and the stator. This phenomenon occured because the flexible rotor had a large radial compliance, and it could undergo radial shear deformation along with the stator deformation in the contact zones. Secondly, owing to the tangential vibration characteristics of the rotor, the braking subzone far from the traveling wave end disappeared, and the length of the tangential shear deformation of the friction layer increased; this change greatly reduced the tangential slip, which leaded to a decrease in the friction loss. These results were expected to contribute toward a better understanding of the dynamic contact mechanics in the TRUM and to guide the design of the friction interface.