Microstructural effects on the microwave dielectric properties of (Ba 0.4 Sr 0.6 )TiO 3 (BST) polycrystalline ceramics were investigated, focusing on the grain size. Sintering temperatures between 1350°C and 1500°C have a strong effect on the permittivity (880 < ε r < 990), quality factor (570 GHz < Q×f < 1150 GHz), and temperature coefficient of resonant frequency (3920 ppm/°C < τ f < 4560 ppm/°C) at microwave frequency (≈1.4 GHz). The tunable permittivity characteristics (measured at 10 kHz), were also found to be sensitive to the sintering process, demonstrating the possibility of tailoring material property by designed processing. In addition, the effect of the sintering process on grain structure was investigated by XRD calculation and Raman scattering characterization. Less confined phonons were believed to contribute to the enhanced microwave performance as an intrinsic effect (grain size effect), for samples with higher sintering temperature and longer dwell time. On the contrary, the macroscopic properties tend to saturate (or deteriorate), for samples at intensified sintering condition, being thought to be dominated by the extrinsic factors (such as the abundant defects in large grains), as confirmed by the SEM observations.