Barium titanate (BaTiO3) fine particles were prepared using the 2-step thermal decomposition method of barium titanyl oxalate. At the 2nd step of this method, the intermediate compound (Ba2Ti2O5·CO3) was decomposed into BaTiO3 and CO2 under various degrees of vacuum pressure. As a result, the particle size of prepared BaTiO3 nanoparticles decreased with decreasing pressure. Moreover, the dielectric constants of these BaTiO3 nanoparticles were measured using the powder dielectric measurement method using slurry. The dielectric constant of BaTiO3 nanoparticles increased with decreasing pressure at the same particle size. It is considered that mesoscopic particle structure controlled by vacuum pressure is important for the dielectric properties of BaTiO3 nanoparticles.