The maintaining of the chemical composition and electrical insulativity of SrTaO2N ceramics was investigated during sintering and annealing, using powders prepared by the nitridation of Sr2Ta2O7. Due to the low thermal stability of SrTaO2N, the partial loss of SrO and nitrogen induced the formation of a TaO0.9 impurity after heat-treating at above 1100°C. The sintering additive SrCO3 and postannealing in NH3 were employed to compensate for the loss of SrO and nitrogen to obtain ceramics with the original chemical composition. The as-sintered SrTaO2N ceramics with various relative density (RD) were annealed in NH3 to observe the recovery of color and electrical insulativity. It was found that the inner part of the well-sintered samples with RD = 95.1% could not be recovered by annealing, and continued to exhibit semiconducting behavior and a black color. On the other hand, for the as-sintered SrTaO2N ceramics with RD < 84%, both the nitrogen content and electrically insulating behavior were completely recovered after annealing. The postannealed SrTaO2N ceramics (RD = 83.3%) possessed a relatively large dielectric constant of 450 with a low dielectric loss of less than 0.1 at 100 Hz, almost independent of frequency and temperature.