Diffraction property of Williams domain grating has been investigated in this work. A liquid crystal (LC) cell is constructed with two indium tin oxide glass substrates and the cell gaps are 38 and 50 £gm, respectively. After filling LC material with negative dielectric constant into the empty cell a good Williams domain grating can be obtained via applying a proper AC voltage. The periodically rolling vortex of LC molecule due to the applying voltage could lead to a Williams domain grating structure. We use a polarized white light source ranged from 400 to 800 nm to probe the grating. The zero-, first-, and second-order diffraction efficiency of the gratings are detected by using a spectrum-meter. It is noted that the diffraction efficiency is a function of wavelength and that is dependent on the polarization angle. The diffraction efficiency is a sinusoidal-like curve in the spectrum. Based on Raman-Nath¡¦s diffraction theory, the diffraction efficiency is an oscillating function of phase difference. The experimental result is in agreement with the theoretical analysis. The dependence of wavelength and polarization on diffraction property may be employed as a light splitting device.