Surface etching of a MgO-doped LiNbO3 crystal is demonstrated by annealing a sandwich structure formed by Mg:LiNbO3/Ti film/Mg:LiNbO3 at 1050-1100 degrees C in a wet O-2 atmosphere. The etching depth, raised Ti layer height and surface roughness were studied as a function of initial Ti film thickness, annealing temperature, and duration. The results show that the maximum etching depth can reach 260 nm while the surface roughness is <5 nm. The features of the raised Ti layer height and surface roughness of the etched surface and raised Ti layer are discussed on the basis of the Ti diffusion mechanism. Combined studies of X-ray single-crystal diffraction, Raman scattering, energy-dispersive X-ray, waveguiding characteristics, surface refractive index, and secondary-ion mass spectrometry show that the etched surface not only retains the LiNbO3 crystalline phase and the original crystal composition but is also Ti-free. An etching mechanism based upon an electrochemical reaction process is proposed. The MgO doping effect on the etching rate and surface roughness is discussed and explained in terms of the MgO doping effect on Ti solubility. A comparison of the +Z and -Z surface etching is made. Finally, the present method is compared with other etching techniques and its possible application is suggested. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3458861] All rights reserved.