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A series of (BaO, Na2O)–Nb2O5–SiO2 glass–ceramics with a varying ratio of Ba/Na have been prepared via controlled-crystallization route, and the phase evolution, and dielectric properties have been investigated. The correlation between the dielectric breakdown performance and the activation energy was studied by the measurements of the dielectric breakdown strength and impedance spectroscopy. It was found that the change in the Ba/Na ratio in the glass notably modified the dielectric properties. The increase of Ba/Na ratio promoted the crystallization of Ba2NaNb5O15 from the glass matrix that has a high dielectric constant. When the ratio of Ba/Na was 2, the optimization of dielectric breakdown performance and dielectric constant was obtained. The as-devitrified (4BaO, Na2O)–Nb2O5–SiO2 glass–ceramic exhibits a large dielectric breakdown strength of 864kV/cm with a stable dielectric constant of about 150 over the temperature range of −50°C to 100°C. [ABSTRACT FROM AUTHOR]