The core-shell structure has offered new possibilities as a multifunctional structure in various fields, such as tunable electromagnetic properties, surface modification, and particle encapsulation. Metal oxide to metal oxide core-shell nanoparticles have especially been intensively investigated for various applications such as dielectric ceramics, energy storage materials, and catalytic nanoparticles. Within the core-shell structure, the shell layer plays a critical role in determining its functionality and properties. In this study, gradient evolution of crystal structure, phase, and composition was studied from nanoparticle surface to its core. By hydrothermal synthesis, a gradient BaTiO3-Ba1-xSrxTiO3 core-shell nanoparticles were synthesized. The core-shell nanoparticles are characterized by scanning electron microscopy, transmission electron microscopy with energy-dispersive X-ray spectroscopy, and synchrotron X-ray diffraction to identify and understand the gradient, multiple Ba1-xSrxTiO3 phases of the shell, which would provide a fundamental understanding of the gradient core-shell structure for future applications.