The shell thickness of a BaTiO3 ceramic with a core-shell structure has been measured by means of synchrotron X-ray diffraction (XRD). BaTiO3 ceramic is known from transmission electron microscope (TEM)/energy dispersive X-ray spectroscopy (EDS) observations to have an inhomogeneous microstructure with cores of a pure BaTiO3 and shells doped with additives. It is also known, from XRD observations, that the BaTiO3 cores have a tetragonal lattice structure and the shells are pseudocubic. We have estimated the shell thickness d from the full-width at half-maximum (FWHM) of the cubic (400)(c) peak, using Scherrer's equation. The shell thickness d(cal) was also evaluated from the volume fraction of tetragonal BaTiO3 using a spherical core-shell model. The two values thus determined agree well, confirming that the BaTiO3 ceramic specimens have a core-shell structure. Our results show that synchrotron XRD is a simple and effective tool for quantitative analysis of the core-shell structure. It enables us to understand quantitatively the relationship between the microstructure and the dielectric properties of BaTiO3 ceramics.