The theoretical behavior of two linearly polarized (VV and HH polarization states) radar backscattered fields is examined from a statistical point of view. The coherency between two polarized signals is studied in terms of their statistical behavior on the phase difference between two signals. The probability density function corresponding to the phase difference between two signals is derived and studied in terms of the degree of polarization and the variance of each polarized signal. The phase difference probability density function (pdf) is then considered with respect to the polarimetric SAR data collected over rugged terrain and ocean. It is found that the phase coherency between two co-polarized channels (VV and HH polarizations) strongly depends upon the degree of terrain roughness structure.