Abstract: Ultrasonic motor (USM) is a stronglynonlinear system with mechanical-electric coupling and closed-loop controls,especially in case of a powerful input. Four types of nonlinearities existin such as a system. (1) nonlinearity due to the stator-rotorcontact; (2) the material nonlinearity of the stator; (3) slow-changingfactors due to temperature and wearing; (4) nonlinearity in the drivingcircuit. These nonlinear factors are related with various nonlinear phenomena, such asjump, hysteresis, drift of resonance frequency, dead zone and saturation.Furthermore, they lead to difficulties in the modeling of USM systems.Asclosed-loop controls of USM require valid dynamic models, thestudy of the modeling of USM systems becomes an important area in theresearch of USM. In this paper, common nonlinear factors in USM systems andtheir modeling methods are reviewed; published studies in the field ofnonlinear dynamics in USM systems are summarized. At the end of the paper,some trends on nonlinear dynamics of ultrasonic motor are discussed.