The relation between the diffraction efficiency of volume grating written by two wave mixing in Ce:KNSBN photorefractive crystal and writing beam parameters such as polarization and beam ratio was studied. The phenomenon that the diffraction efficiency strongly depends on writing beams polarization was analyzed, and the modified coupled-wave theory was used to fit the experimental data. The results give a powerful support to the application of Ce:KNSBN crystal in holographic recording and optical information process.