Barium titanate (BaTiO3) ceramics with a high-density were fabricated by two-step sintering method from hydrothermally synthesized 100 nm BaTiO3 nano-particles. The best specimen with an average grain size of 1.6 μm and a density of 5.91 g/cm3 (98.3% of the theoretical value). The dielectric constant was 4500 and electromechanical coupling factor kp was 45%. Large piezoelectric constants d33 = 460 pC/N and d31 = -185 pC/N were observed in the specimens. This was an important practical result towards obtaining a high d33 in non-lead-based BaTiO3 ceramics manufactured by a low-cost process. These results also indicated the possibility of using BaTiO3 ceramics in piezoelectric devices at room temperature. Temperature dependence of dielectric constant showed two peaks located at 24 and 126 oC, corresponding to orthorhombic-to-tetragonal phase transition temperature Tot and Curie temperature Tc, respectively. Owing to the size effect of nanocrystals, Tot shifted to 24 oC. The maximum of electromechanical coupling factor kp appeared close to the phase transition temperature. It also caused a very large temperature coefficient of resonance frequency from room temperature to 60 oC. Hysteresis curve measurement showed a very low coercive field Ec = 115 V/mm. A large Poisson’s ratio, about 0.38, was determined from the ratio of overtone frequency and resonant frequency in the planar mode. The high Poisson’s ratio and the large dielectric constants are most likely the origin of the high piezoelectric constants in the ceramics.