The ultrasonic nondestructive technique is mainly used to evaluate interior defect, material properties and outside dimensions by measuring the transmitting and reflecting sound waves. Generally, the evaluation of the ultrasonic testing depends on the amplitudes and delay time of the received signals; however, this research is focused on the analysis of the phase differences of the received signals. The leakage phenomenon of surface waves propagating at the liquid-solid interface has been studied for more than fifty years. The main characteristic of this phenomenon is the 180-degree phase difference between the reflected and leaky ultrasound when a bounded ultrasonic beam is incidented. And the null zone caused by the interference of these two waves is appearing in the reflected field. The phase difference is changed as the surface condition altered, including surface roughness and layered structures. The normal-mode theory is used at this research to analyze the analytical model of the leaky surface wave in thin layered structures. In experiments, the measurements of the reflected field are proceeded by the scanning system, so as to analyze the phase difference between the reflected and leaky ultrasound and calculate the layer thickness by the phase difference. As a result of the surface roughness of the thin layers, the leakage is more serious when the ultrasound propagates with lower frequency. For the consideration of locating the null-zone in the reflected field, this research prefers using lower frequency as an initial frequency at the beginning of the testing, then increasing the frequency to achieve a better sensitive of the thickness.