Most power ultrasonic applications use Langevin transducers to generate power ultrasounds. In this chapter, we focus on a model for resonant power transducers under extreme excitation conditions. The challenge here is to correctly consider the usual resonance nonlinearities, such as jump phenomenon of transducer, asymmetric resonance peaks, and diminution of mechanical quality factor. The mechanical equation of the piezoelectric element is written using electrostriction. The dielectric part is written as a nonlinear function of an equivalent electric field including stress influence. Good simulation results allow concluding that in an ultrasonic transducer nonlinearities are only due to dielectric nonlinearities.