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.