The ultrasonic technique is one of the most promising methods to nondestructively evaluate the interface. When the adhesive bonding is not perfect, part of ultrasonic energy is reflected from the interface. The reflection signal can be used to estimate the adhesive strength. In this investigation, a spectral analysis is employed using Fast Fourier Transform (FFT) to evaluate the reflection signal in frequency domain. The frequency spectrum of the reflection signal is then deconvolved with a reference signal taken from the bottom of the test block to determine the interfacial stiffness which can provide useful information on the nature of the adhesive interface. In this work, two aluminum blocks are adhesively bonded with the epoxy. The adhesive bond strength measured by a tensile testing is correlated with the interfacial stiffness. Experimental results show that the adhesive bond strength is linearly increasing with the increase in interfacial stiffness. It demonstrates that the adhesive bond strength can be characterized using the single parameter of interfacial stiffness. The present study reveals the feasibility of assessing the adhesive bond strength with the ultrasonic technique.