The electron paramagnetic resonance (EPR) technique was employed to detect oxygen vacancy defects in the tetragonal Ba1− x Ca x TiO3 (x = 0.03) ceramics (BCa3T) prepared via the mixed oxide route at 1300–1500 °C. In the rhombohedral phase below −100 °C, an EPR signal at g = 1.955 appeared in the insulating BCa3T with an electrical resistivity of 108 Ω cm and was assigned to ionized oxygen vacancy defects. BCa3T prepared at 1300 °C showed a temperature-stable X6S dielectric specification (ε′ = 1750). Three types of vacancy defect, namely, Ba, Ti, and O vacances, could coexist in BCa3T owing to the partial Ti-site occupation by Ca2+.