Nb-doped 0.9BaTiO3–0.1Bi0.5Na0.5TiO3 (0.9BT–0.1BNT) ceramics were prepared by conventional solid-state method. The dielectric and the structural properties were investigated. It was found that the temperature–capacitance characteristics greatly depended on Nb2O5 content. With the addition of 2.0 mol% Nb2O5, 0.9BT–0.1BNT ceramic sample could satisfy the EIA X9R specification. This material was promising for high-temperature MLCC application. Microstructure element distribution was studied using TEM and EDS. The Bi and Na were almost homogeneously distributed except grain-boundary segregation of Bi. The Nb exhibited a nonuniform distribution from the grain boundary to the interior, showing the simultaneous presence of Nb-rich and Nb-poor regions. Such microheterogeneity gave rise to the temperature stability of permittivity. The solution-precipitation mechanism was introduced to elucidate the evolution of microstructures. Degradation and recovery of insulation resistance were observed under a dc bias at 200°C, which was attributed to the electromigration and diffusion of Na+.