Polarization mode dispersion (PMD) causes severe signal impairments in high bit-rate optical transmission systems. It is important to know how does the degree of polarization (DOP) change with differential group delay (DGD) when DOP is feedback information for adaptive PMD compensation. The mathematical expression between DOP and DGD is derived for arbitrary waveshape and Gaussian pulse. Then the relationship of DOP-DGD with different power splitting ratio, width of pulse, and amount of pluses is theoretically analyzed. Experiments with 10 Gbit/s return-to-zero/non-return-to-zero (RZ/NRZ) pseudo random sequence are conducted and comparison between theoretical calculation and experimental results is made. The comparison showed that they matched closely. It is also verified that DOP was independent of chirp and fiber dispersion in experiment, which made the theoretical analysis of DOP-DGD easy.