The ion valence state, phase composition, microstructure, and microwave dielectric properties of Sr(1−1.5x)CexTiO3 (x = 0.1–0.67, SCT) ceramics were systematically investigated. Sr(1−1.5x)CexTiO3 ceramics were produced with gradual structural evolution from a cubic to a tetragonal and turned to an orthorhombic structure in the range of 0.1 ≤ x ≤ 0.67. Above a critical Ce proportion (x = 0.4), microstructural changes and normal grain growth initially occurred. On the basis of chemical analysis results, the reduction of Ti4+ ions was hastened by tetravalent ions (Ce4+). By contrast, this reduction was inhibited by trivalent ions (Ce3+). The observed dielectric behavior was strongly influenced by phase composition, oxygen vacancies (VnormalO∙∙), and defect dipoles, namely, (Ti′−VO∙∙) and (VSr″−VO∙∙). Temperature stable ceramics sintered at 1350°C for 3 h in air yielded an intermediate value of dielectric constant (εr = 40), with the smallest reported value of temperature coefficient of resonant frequency (τf = +0.9 ppm/°C), and quality factor (Q × f = 5699 GHz) at x = 0.6.

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