The polarization states of the diffraction beams of nearly perfect conducting blazed gratings in Littrow mounting are studied by numerical simulation.A FDTD program is written,the electromagnetic fields for various polarized incident beams are calculated,and the polarization states of the diffractive waves are extracted.The relation between the polarization state of the diffraction wave and the grating balzed angle and polarization angle for the linearly polarized incident wave is analyzed.In the spectral range of 1 530 ~ 1 570 nm,the polarization dispersion is studied for the linearly and circularly polarized incident beam.The most important conclusions may be significative for the engineering design.