Ultrasonic surgical instruments often suffer from significant performance variation because of the high power applied and the physical loading from soft tissue encountered in application. To study these effects in detail, an ultrasonic planar tool coupled either in the air or poultry breast tissue was driven by a bespoke resonance tracking and vibration stabilization system at different current amplitude levels, until the performance achieved the steady state, during which two frequency sweeps were conducted at the beginning and the final state, respectively. Results show that the electric impedance increased rapidly with current amplitude, partly resulting from the contact resistance in the electric circuit in the tool, which in turn generated intensive self-heating at high power. The resonant frequency shifted to lower values because of the high vibration velocity and the temperature increase of the piezoelectric material. The radiation resistance induced by the soft tissue loads increased with the coupling depth of the planar tool but decreased with the current levels.