Abstract: The groove misalignment of the grating-pair compresor does affect its dispersion characteristics.The diffraction characteristics of grating-pair compressor is analyzed under the condition that the grating surfaces are parallel while the grating grooves are unparallel,and the optical path length of pulse passing through this system is derived.Then the second-order,third-order and fourth-order dispersions are calculated.The resulting relative dispersion errors compared with that the grating grooves are strictly parallel are shown.The vertical spectrum displacement in the output pulse is calculated and shown quantitatively as function of the angle between the grooves.The results show that gratings with low groove density are superior to that of high density ones,if the groove misalignment is unavoidable.