Polarimetric SAR (POLSAR) data contains both phase and amplitude information from radar returns transmitted in four different polarizations. Thus it is necessary that we place strong emphasis upon extracting physical information from the observed scattering of microwaves. In this paper, we studied the Freeman decomposition method as well as some feature parameters that can represent targets' characteristics. Further more, we made an in-depth investigation on applying these parameters to discriminate several representative terrain cover types.