A fully polarimetric borehole radar system with four combinations of dipole and cylindrical slot antennas was developed to acquire fully polarimetric data sets in drilled boreholes. To better under the fully polarimetric response to subsurface fractures with different roughness, in this study, synthetic fractures with different roughness are generated on a computer via fractal theory based simulation techniques. Quantitative assessment for the roughness of synthetic fractures is possible by use of three main parameters: the fractal dimension, the rms roughness at a reference length, and a length scale describing the degree of mismatch between the two fracture surfaces, allowing future detailed study of mechanical and transport properties of fractures and fully polarimetric radar response on them. Next, a 3D sub-grid FDTD numerical simulation is used to synthesize fully polarimetric data sets with synthetic fractures as primary reflectors. Based on the synthetic data sets, it is possible to relatively quantitatively evaluate the applicability of different radar polarimetry analysis approaches to physical characterization of subsurface fractures.