The polarization mode dispersion (PMD) of fiber Bragg gratings (FBG) is detailedly studied. Theoretic analysis and experimental measurement are carried out on FBG. Birefringence of UV written gratings mainly comes from the asymmetrical absorption of photosensitive fiber, or exponential absorption. Considering the influence of UV induced fiber birefringence, gratings chromatic dispersion on the PMD of FBG, the model of FBG's PMD is founded. Numerical calculation is concident with the experimental measurement. The dispersion of FBG can effect PMD.