We calculate the velocity surface of bulk acoustic waves propagating in the YZ-, XY-and ZX-planes of proton-exchanged LiNbO 3 , based on the theory of acoustic wave in anisotropic solid with piezoelectricity. The calculation results show that the velocity change due to proton-exchange when the wave propagating along X-axis is larger than that along Z-axis for Y-cut substrate, in spite of longitudinal or transverse wave. The curve of velocity surface for PE Z-cut substrate is near to a circle, while that for pure Z-cut LiNbO 3 is a hexagonal. The velocity surface for PE X-cut LiNbO 3 seems to be turned an angle from that for pure X-cut LiNbO 3. The detail results, discussions and comparison with the experiment will present in this paper.