BaTiO₃ ceramics with different domain sizes were prepared by spark plasma sintering (SPS) and normal sintering (NS) using hydrothermally synthesized 100 nm and 500 nm powders. It was found that their piezoelectric constant d₃₃ shows great dependence on the size of ferroelectric domains. A high d₃₃ up to 416 pC/N was obtained in the SPSed ceramics derived from 100 nm powders, whose domain width was identified <50 nm by transmission electron microscopy (TEM). However, the d₃₃ values are 216 and 193 pC/N, respectively for the BaTiO₃ ceramics prepared by SPS using 500 nm powders and by NS using 100 nm powders, whose microscale domains can be observed by optical microscopy.