Surface diffraction gratings are written on LiNbO3 crystal by femtosecond laser pulses with the wavelength of 800 nm, pulse duration of 50 fs and repetition frequency of 1000 Hz. The diffraction efficiencies of the gratings ablated with various parameters are measured by He-Ne laser with the wavelength of 632.8 nm. The 1-order diffraction efficiencies of the gratings improve from 1.7% to 2.3% with the increase of ablation speed from 20 μm/s to 200 μm/s, and decrease from 1.9% to 1.3% with the increase of pulse energy from 70 nJ to 110 nJ, respectively. The experimental and theoretical results show that the diffraction efficiencies of gratings can be improved by increasing ablation speed and grating constant or decreasing pulse energy.