A nematic liquid crystal (LC) grating is prepared using an amplitude-type grating electrode structure and a homogeneous initial alignment parallel to the grating electrode direction. The polarization properties of the LC grating are investigated by measuring Stokes parameters of diffracted light under differently polarized incident light. The electrically variable polarization modulations of diffracted light are verified for the first time, as well as the diffraction light intensity modulations. Moreover, the LC grating exhibits some unique optical properties, such as symmetric diffraction light intensities but antisymmetric polarization states in corresponding positive and negative diffraction orders when the incident light is linearly polarized parallel or perpendicular to the grating direction. A polarization grating model is proposed to clarify the unusual polarization properties, which suggests that it may be related to the antisymmetric LC molecular orientation domains induced by the grating electrode structure.