By manipulating process parameters, BaTiO3 nanoparticles with tunable size were successfully prepared by flame-assisted spray pyrolysis (FASP) from an aqueous solution of barium acetate and titanium-tetra-isopropoxite. Particle size was controlled over a wide range (from about 23 to 71 nm) by varying the concentration of precursor and methane flow rate. Flame temperature was a key factor in producing particles with a narrow size distribution. The BaTiO3 nanoparticles were cubic in crystal structure, dense, spherical and softly agglomerated. The particles contained OH, carboxyl and CO2 bonding groups that could be completely removed by post-heat treatment. At room temperature, BaTiO3 pellets had relatively high dielectric constants (2578.8–3611.8) with loss factors ranging from 2.6% to 7.1% at the frequency of 1 kHz. The results of this study indicate that BaTiO3 nanoparticles can be fabricated using continuous and industrially applicable FASP.