The sintering behavior of 0.36BiScO 3 -0.64PbTiO 3 (0.36BS-0.64 PT) ceramics was studied to investigate the effect of grain growth by the sillenite Bi 12 PbO 19 (BP) phase on their piezoelectric properties for application in high-temperature piezoelectric devices. The BP phase formed during calcination at temperatures <750 °C led to a grain growth anomaly of the 0.36BS-0.64 PT ceramics sintered at 1000 °C. This phase assisted the grain growth of the 0.36BS-0.64 PT ceramics by liquid phase sintering. In particular, the 0.36BS-0.64 PT ceramic calcined at 700 °C exhibited excellent piezoelectric properties with a d 33 of 531 pC/N, g 33 of 41×10 −3 Vm/N, k p of 61.8%, and Q m of 16. In addition, the 0.36BS-0.64 PT ceramics exhibited ferroelectric relaxor-like characteristics with an extremely large relaxation coefficient ( γ ) of 1.94 along with high maximum dielectric permittivity temperature (426 °C).