In order to understand the fundamental physics of the heat generation at a defect in Sonic-IR method further, and to discuss the suitable ultrasonic excitation method to improve the detectability of defects, Sonic-IR tests for the strip shape specimens with single edge penetrating fatigue crack were carried out with changing the ultrasonic excitation position, and the relationship between the vibrational modes of the specimen and the temperature rise at the fatigue crack vicinity was investigated. Three specimens with different crack locations were prepared. In all specimens, the temperature rise at the fatigue crack changed periodically with the change of the ultrasonic excitation position. For each specimen, the 2nd, 4th and 6th order torsional natural vibrations were observed, and among these vibrations, there was a torsional vibration which strongly affected the heat generation at the crack. The position of the anti-nodes and nodes of such vibration corresponded well to the position of the peaks and the bottoms of the temperature rise, and the order of the vibration varied depending on the location of the crack. It was found that the torsional vibration that strongly affected the heat generation at the crack had the maximum vibration energy calculated from the relative displacement between the two crack surfaces and the frequency of the vibrations among the three vibrations observed in the specimens. In the conditions of the present study, the presence of a crack at the nodal position could increase the relative vibration between the two crack surfaces due to the vibration of these two crack faces in opposite phases, thus increasing the vibration energy, i.e., the heat generation at the crack.