Ferroelectric local domain inversion occurs in LiTaO3crystals by proton exchange followed by heat treatment at temperatures just below the Curie point, thereby yielding an inversion layer at the −csurface. This article reports the domain inversion phenomenon in LiTaO3plates and the experiments performed to clarify the mechanism. Measurements of dielectric constant and electrical conductivity near the Curie temperature suggest that the pyroelectric fields induced during the cooling down period are not likely to be the main cause of the domain inversion. This experimental result is reinforced by measurements of the piezoelectric response during heat treatment, which shows that the antipolarity domain nucleates at the initial stage of heat treatment and grows gradually.