Ground penetrating radar (GPR) waves in inhomogeneous medium are analyzed by numerical method in this paper. The theory analysis provides physical interpretation of a calculation model for characteristics of subsurface layers. In this model, the thickness and permittivity of subsurface layers would be estimated by traveling time of reflection wave in GPR measurement. A 3-D finite-difference time-domain (FDTD) code is used to simulate electromagnetic fields in multi-layer ground. Numerical simulations of the responses of GPR are described. Responses of different offset radars are studied in this paper. The thickness of planar layer is calculated by simulated results. Results calculated from FDTD simulations verify the accuracy of the model for estimating the parameters of ground by GPR.