Anisotropic diffraction of gratings in photorefractive crystals can be used to improve the signal to noise ratio of readout signals from photorefractive memories.In this paper the polarization properties of formation and diffraction of gratings in LiNbO 3 crystal are analyzed based on the susceptibility tensor and the effective electrooptic coefficients.Experiments on the case when the caxis is coplanar with the grating vectors are conducted,showing that the anisotropic diffraction is obvious and the measurement of readout angle for anisotropic diffraction is agreed with theoretical calculation.