We developed deep-proton exchanged (DPE) and annealed waveguides in MgO:stoichiometric-LiTaO3 crystal. We performed DPE in pyrophosphoric acid at 230 °C for 20 min and measured mode size and depth dependent on the annealing duration. High-quality planar waveguides of a mode size as large as 20 µm, suitable for electro-optic spatial light modulators for 355 nm UV wavelength, were obtained by annealing for 200 h.