Second‐harmonic generation in a LiTaO3 periodically poled by electric field is presented. The side growth of the domain‐inverted region during the poling was found to be suppressed using selective proton exchange. Utilizing this technique, a first‐order periodic domain inversion having uniform periodicity was realized over interaction length of 10 mm in a 300‐μm thick LiTaO3 substrate. A 3.8 mW of harmonic blue light was generated for a conversion efficiency of 1.27% in a single pass through the periodically domain‐inverted structure.