Abstract: The Fokker bond tester has a long history of being developed and widely used. However, some analysis, taking into account the whole system and more or less quantitative, on its working principle does not yet seem available. In this paper, a phenomenological explication of the working principle is obtained by computing the loading of the transducer by the induced vibration of the bonded specimen and checking up with experiments. In the analysis, simple models and mathematical approximations are introduced. The somewhat crude theory is supported by the approximate quantitative agreement between the theoretical predication and the experimental data.