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The grain size influence on the dielectric relaxation and nonlinear dielectric properties of BaTiO3 ceramics with grain size in the range of (92÷936) nm densified by Spark Plasma Sintering (SPS) from ultrafine powders were investigated. The progressive reduction of the Curie temperature and of the effective permittivity results from a combination of intrinsic size effects and low-permittivity grain boundary layer. A model of dielectric cylindrical cavities was employed in order to calculate intrinsic effective permittivity values in GHz range. An interesting feature is the presence of a thermally activated Debye-like relaxation in the ferroelectric state of ceramics with grain size above 300 nm, with activation energies of 0.45–0.49 eV, which seems to be related to the domain walls forced motion under the applied field. By diminishing grain size, a progressive reduction of the ferroelectric nonlinear character was obtained, reaching a macroscopic non-switching character and a linear permittivity versus field dependence for the finest ceramics (grain size of 90–100 nm) until very high values of the applied field. The observed behavior supports the idea of frozen polarization induced by pinning centers as due to a large number of grain boundaries and charged defects in the fine structures.