We introduce applications of radar polarimetry in borehole radar. One example is classification of subsurface fractures, where we estimate the water permeability of each fracture by radar polarimetry. Then polarimetric borehole radar was then used for cross-hole radar measurement. Transmitted signal between boreholes having a 20 m separation was measured, and we could show that the polarization information can be used for estimation of the orientation of a subsurface cavity.