Method for fabricating proton-exchanged LiTaO3 optical waveguides in pyropho-sphoric acid below 300℃ was reported for the first time. The effective diffusion coefficient is measured as D=D0 exp(-Ea/kT) with Do of 3.91×106μm2/h and the activation energy Ea of 0.74V, and waveguide parameters were also measured. The experimental results show that only the extraordinary index is increased and the propagation loss in the TM single-mode waveguides as low as 0.1 dB/cm at 633 nm. It is found that the surface extraordinary index is increased after annealing.