The result of a systematic study on planar waveguides fabricated in z-cut LiNbO3 by proton exchange and the post-annea ling is reported. The diffusion constant and the activation energy can be deduc ed from the proton exchange process. It was found that index profile of the as -exc hanged wavegudies is a linear step profile. The index profile changes from step -like to Gaussian profile with succesive annealing. The annealing depth increase s exponentially with annealing time at the first stage and then increases linearly with the square root of annealing time at the second stage of annealing.