The anisotropic diffraction property of photorefractive gratings has been studied quantitatively. The dependence of the efficiency of anisotropic diffraction in LiNbO3 crystals on the beam polarization and recording configuration is theoretically calculated based on two constrains for remarkable anisotropic diffraction, the phase matching and non-zero effective electro-optic coefficients. Then numerical calculations have been conducted based on the theoretical model. It is found that the anisotropic diffraction efficiency is remarkable and easy to be observed in transmission gratings written by two extraordinary beams with the crystal axis perpendicular to the plane of writing beams. The results of experimental measurements of the readout angle for observing anisotropic diffraction agreed well with the theoretical calculations. Some experimental results of anisotropic diffraction efficiency also agreed with the theoretical calculations.