An evaluation technique of internal boundary surface using instantaneous frequency of ultrasonic pulse is proposed. The instantaneous frequency represents a variation of the frequency of the ultrasonic pulse waveform from moment to moment. In this paper, a novel technique to acquire precisely the instantaneous frequency of ultrasonic pulse is proposed. Results of fundamental experiments, in which several internal boundary conditions are simulated by rubber elements, show that the instantaneous frequency of ultrasonic pulse transmitted the internal boundary is sensitive to the change of the boundary condition. Profiles of instantaneous frequency significantly change according to surface roughness and contact pressure of boundary, so that the internal boundary conditions can be evaluated by the instantaneous frequency. It is demonstrated that internal boundary conditions can be modeled as linearly or non-linearly change of partial stiffness, and the profiles of the instantaneous frequency depend on the partial stiffness. In addition, it is shown that the proposed method is not only available to rubber element but also metallic component.