Barium titanate/polyimide (BaTiO3/PI) nanocomposite films with high dielectric permittivity (20), high breakdown strength (67 MV m−1), and high thermal stability are prepared by an in-situ polymerization process. A very thin polymer layer (about 5 nm) is coated on the surface of nanosized BaTiO3 particles to form a core–shell-like structure, which can guarantee homogeneous dispersion of the BaTiO3 particles in the PI matrix. It is confirmed that the core–shell-like structure originates from both the electrostatic attraction between the precursor poly(amic acid) (PAA) and the BaTiO3 particles and the hydrogen bond interaction between PI and the BaTiO3 particles. Such a structure also has some influence on the dielectric properties and breakdown strength of films. After casting and degassing of the sticky film, the dielectric permittivity of the nanocomposite film is close to or even higher than that of submicrocomposite films, which is attributed to the advanced interfacial structure between the BaTiO3 and PI phases.