Photorefractive diffraction gratings were studied in cells of homeotropicall-y aligned 4-cyano-4-pentyl-biphenyl (5CB) liquid crystals doped with buckminsterfullerene (C60) based on two-beam coupling (TBC) experiment. In the 20 μm thick NLC sample (grating spacing was Λ≈24 μm), the first-order diffraction efficiency of the orientational PR grating was as high as 40%, and the distribution of the diffractive intensities was asymmetric. According to the volume-mediated photorefraction and surface-mediated photorefraction, a qualitative analysis that the electric field enhancemeat results from the surface charge modulation and Carr-Helfrich effect is proposed. The high first-order diffraction efficiency obtained from the thin grating in NLC is probably attributed to the non-sinusoidal modulated space-charge field. The simulation results are in good agreement with the experimental results and also confirmed the physical mechanism.