A thermal diffusion process of Ti into a LiNbO3 substrate for optical waveguides has generally been carried out under a wet gas atmosphere in order to prevent undesirable Li outdiffusion. In this work, such thermal decomposition was confirmed to be significantly suppressed for an OH-free LiNbO3 substrate, even after a dry atmosphere annealing. No extra x-ray diffraction peak for LiNb3O8 was detected from the OH-free substrate after 10 h of annealing at 1000 °C in a dry O2. Furthermore, the surface morphology of this sample, and as well an unannealed one, were smooth. In a conventional LiNbO3 substrate containing many OH ions, subjected to a similar dry annealing, the presence of the LiNb3O8 phase and a surface coarsening were observed.