In this paper, we systematically investigated the effect of microwave sintering parameters on the characteristics of BaTiO3 capacitor materials co-doped with Y2O3/MgO species. It is observed that the granular structure of the materials is relatively insensitive to the sintering temperature and soaking time such that the BaTiO3 capacitor materials possessing X7R dielectric constant-temperature (K-T) characteristics can be obtained in a wide range of sintering conditions. TEM examinations reveal that the detailed microstructure of these materials is extremely complicated. The unique K-T properties of these materials are ascribed to the duplex structure of the samples, viz. fine grains of paraelectric phase and large grains of ferroelectric phase.