In order to understand the fundamental physics of the heat generation of defect vicinity in Sonic-IR method, and to discuss the suitable excitation method to improve the detectability of defects, Sonic-IR tests under various excitation conditions were carried out, and the relationship between the vibrational state of a specimen and the heat generation characteristics of a fatigue crack vicinity was investigated. By ultrasonic excitation in the frequency of 21 kHz, the vibration at 21 kHz and its higher harmonic vibrations at 42 kHz and 63 kHz were observed. For the change of the ultrasonic excitation position, the temperature rise at the fatigue crack changed periodically. The peak and bottom positions of the temperature change corresponded to the positions of the antinode and the node of the 6th mode torsional vibration. It was found that the heat generation of the crack vicinity strongly affected by the 6th mode torsional vibration, because the vibration energy of the 6th mode torsional vibration was larger than the other mode torsional vibrations.