Barium titanate (BaTiO3) nanoparticles were prepared by a two-step thermal decomposition method using barium titanyl oxalate nanoparticles of size 30 nm with and without dry-jet milling. Dry-jet milled barium titanyl oxalate nanoparticles (BTO-B) were well-dispersed whereas those without the dry-jet milling procedure (BTO-A) were partially aggregated. A heat annealing of obtained BaTiO3 nanoparticles at the same temperature resulted in crystallite sizes of the BTO-A derived BaTiO3 nanoparticles much smaller than those of the BTO-B derived. A mesoscopic particle structure analysis of revealed much thinner surface cubic layer thickness of the BTO-B derived BaTiO3 nanoparticles compared with the BTO-A derived BaTiO3 nanoparticles. This indicated the particle growth rate to be the most important parameter for the surface cubic layer thickness determination. A relationship between the surface cubic layer thickness and the particle growth rate was investigated precisely in this study.