Ground Penetrating Radar (GPR) has been widely applied in civil engineering and environmental detection as a high resolution and efficient nondestructive testing (NDT) method. For example, it is used to detect the rebar concrete structures. In this paper, the FDTD method was used to simulate the GPR detection of the defects in the reinforced concrete and to migrate with the frequency wavenumber (F-K) migration method. This study shows that the technique can be used to reduce the multiple and diffraction of rebar. With the F-K migration method, the diffraction energy is efficiently focused on rebar and defects, which can not only separate the signals of rebar from those of defects but also improve range resolution. As a result, the distribution and shape of defects can be easily interpreted.