Abstract: Void condition of runways is an important factor for runways quality evaluation. Ground Penetrating Radar is widely applied in the quality surveillance of runways. Void-layer detection and thickness determination in runways based on GPR are studied in this paper. Void-layer is thin layer whose thickness is very small (usually less than 1cm). Firstly, the void-layer can be detected based on its time-frequency distribution characteristic of S transform. This is the first attempt of S transform to be used in the void-layer detection of runways. In the same time, the depth of void-layer can be evaluated based on spectrum optimisation inversion method, in which the impulse pair in the time domain corresponding to the upper and bottom interface of void-layer can be extracted based on the improved EM wave propagation model and system identification method we have proposed in other paper. The results demonstrate the method is valid and the estimation error of thickness is within 5%.