Abstract: Ground Penetrating Radar (GPR) can achieve good result in the void detection of airfield runways, but it is still impossible to correctly estimate the void thickness in lossy medium. To solve this problem, based on the amplitude response of complex echoes reflected from thin void layer and the electromagnetic (EM) wave propagation model in lossy medium, this paper extends conventional turning thickness method to lossy medium and proposes a novel thin-layer thickness determination method. Firstly, the amplitude response between amplitude corresponding to predominant frequency and void thickness was deduced. Then the predominant frequency amplitude response was modified by least-square curve fitting method, which can reduce the effects of unmolded error and parameter estimation error to estimation accuracy of void thickness. Simulation results demonstrate that the estimation accuracy of proposed method is obviously higher than that obtained by unmodified method.