Proton-exchanged Z-cut LiTaO_3optical waveguides are fabricated and the their optical properties before and after annealing are studied. The refractive index profiles in the waveguide before and after annealing are discussed and verified in this paper. It is proved that the refractive index profile will be envolved from step type to Fermi type after annealing. Measurement of the waveguide refractive index at 632.8nm wavelength show that the index increment is about 0.012 before annealing and nearly 0.025 after annealing.