The formation of refractive index grating in pure and doped Nematic Liquid Crystal (pure 5CB and doped with SBB) in planar cells has been investigated. A DC voltage is applied to the samples and a He-Ne laser is used as writing beam to form the grating. It is observed that doping the LC with azo dye leads to higher diffraction efficiency with respect to the pure one and lower needed writing beam intensities. These observations are described by orientation of liquid crystal molecules, which can only be detected by P-polarized probe beam.