We investigated the crystal structure of SrRuO3/BaTiO3/SrRuO3 (SRO/BTO/SRO) heterostructures grown on a SrTiO3 (001) substrate using surface X-ray scattering. The interface structure of SRO/BTO/SRO heterostructures can decrease the critical thickness of ferroelectric BTO films reduced to 3.5 unit cells. Owing to weak intensity of ultrathin films, lab-source-based X-ray measurement cannot determine the atomic arrangement of the heterostructures and their lattice parameters. We introduced synchrotron-based X-ray scattering techniques combined with coherent Bragg rod analysis (COBRA) to resolve the details of the heterostructures. We obtained detailed crystal structural information based on the electron density profiles of the SRO/BTO/SRO heterostructures acquired from COBRA results, e.g., lattice parameters and atomic arrangements.