Abstract: Thermosonic imaging is a new kind method of NDE/T technique for detecting surface and subsurface cracks and defects. In the technology, the cracks(defects) will be heated under intensive ultrasonic pulses excitations due to both sides of the cracks have different vibration amplitudes and phases. In the experimental studies of the technology, it has been found that the cracks display obviously different temperatures(heats) while the ultrasonic excitation sources are located at different positions. On the other hand, based on the ultrasonic propagations in plates a theoretical model is presented to calculate the vibrations of both sides of the cracks, and the amplitude and phase differences between two vibrations of both sides of the cracks are obtained, in which the harmonic components are considered. The results show that the differences are dependent on the relative directions and distances between the ultrasonic sources and the cracks. The amplitude and phase differences of both sides of the cracks determine the temperatures (heats) around the cracks. Finally, the results are analyzed and discussed, which show that the theoretical calculations are in agreement with that of the experimental measurements.