To prepare high dielectric thin films of polymer-based materials, nanometre sized BaTiO3 particles, which should have high dielectric coefficients and low energy dissipation factors due to nano-size effects, were dispersed in polyvinylidene fluoride (PVDF) or siloxane-modified polyamideimide (SPAI). BaTiO3 particles with crystal sizes of 10.5-34.6 nm were synthesised with a complex alkoxide method. Polymer/N-methyl-2-pyrrodinone solution suspending the BaTiO3 particles was spin-coated on ITO glass substrates to prepare polymer-based composite films with submicron thickness. The BaTiO3 particles were dispersed more homogeneously in the PVDF film than in the SPAI film. The good dispersion of the particles in the PVDF film brought about a smooth film surface that had a root mean square roughness < 20 nm at a particle volume fraction of 30%. The roughness was less than one-tenth of the roughness of the SPAI composite film. An increase in the BaTiO3 crystal size from 10.5 to 34.6 nm in the PVDF film at a particle volume fraction of 30% increased the dielectric constant of the film from 20.1 to 31.8. The BaTiO3-PVDF composite film attained a high dielectric constant with more than twice the dielectric constant of the BaTiO3-SPAI composite film. The dissipation factor of the PVDF composite film was as low as 0.05 at 10000 Hz. 16 refs.