A novel BaTiO 3 -Na 0.5 Bi 0.5 TiO 3 -Nb 2 O 5 -NiO (BT-NBT-Nb-Ni) system that meets the X8R specification (-55°C-150°C, ΔC/C≤±15%) of multilayer ceramic capacitors (MLCCs) was fabricated, with a maximum dielectric constant of 2350 at room temperature (25°C). Core-shell microstructure was observed by transmission electron microscopy (TEM), accounting for the good dielectric temperature stability. The role of Ni on the formation of core-shell structure and phase structure, and the subsequent relationship between structure and dielectric/ionic conduction properties were investigated. It was observed that the addition of Ni could adjust the ratio of core/shell, and significantly reduces the dielectric loss over the studied temperature range. A new Ba 11 (Ni, Ti) 28 O 66+x phase with a 10-layer close-packed structure was identified by X-ray diffraction (XRD), serving as a source of oxygen vacancies for ionic conduction in addition to Ba(Ni,Ti)O 3 . Furthermore, the impedance spectroscopy measurements demonstrated the remarkable impact of these Ni-induced oxygen vacancies on both the grain and grain-boundary conductivities.