A periodically domain-inverted structure in LiTaO3 has been fabricated for quasiphase-matched (QPM) second-harmonic generation (SHG). Uniform proton exchange is achieved with pyrophosphoric acid. Using this technique with a Ta mask followed by heat treatment, the periodically domain-inverted structure for quasiphase matching is realized over large areas. The effect of fabrication conditions on the domain-inverted profile is investigated to form a periodic structure having a considerable depth and a short inversion period. Consequently, a third-order QPM-SHG device can be fabricated by using this periodically domain-inverted structure and proton-exchanged waveguide. To investigate period regularity of the domain-inverted structure and nonlinearly of the inverted region, phasematching characteristics and conversion efficiency of this device are examined. The formation mechanism of the domain inversion is also discussed.