A planar waveguide is fabricated by 3 MeV O2+ ion-implanted in MgO-doped lithium niobate, and the refractive index profiles of waveguides are reconstructed based on etching and ellipsometry techniques. The SRIM2003 code is used to simulate the damage distribution induced by implantation. The etching rate versus the etching depth is extracted and the relation between the etching rate and the damage profile is discussed. The index profile of this kind of waveguide is determined by etching in combination with the following ellipsometric measurements. Both ordinary and extraordinary index profiles in waveguide are obtained. The influence of damage profile on index profiles in waveguide, as well as that on waveguide properties is analysed.