Thin-film lithium niobate (LN) has been proved to be an excellent platform for building compact active and nonlinear photonic components on a chip. The coupling of a sub-micron sized LN waveguide and a single-mode fiber remains as one challenging issue. An efficient grating coupler made of Au stripes on an LN ridge waveguide is demonstrated here. The fabrication of the grating is convenient, using just a standard lift-off process of metal films. The peak coupling efficiency of an optimized structure reaches 50.4%, i.e., -3 dB coupling loss, at 1.55 µm wavelength for the fundamental transverse-electrical mode, where the 1-dB coupling bandwidth is 58 nm. Experimentally, fabricated devices, with buried oxide layer thicknesses slightly off the optimal values, exhibit coupling efficiencies of 43.8% and 33.7% for 400 nm and 600 nm thick LN layers.