The heterogeneous precipitation method has been successfully used to prepare SrCO3 nanoparticle-coated BTO (BaCO3 or TiO2) powders by using PAA (polyacrylic acid) and NH4HCO3 as the dispersant and precipitator, respectively. Compared to the mixture obtained by the direct ball milling of the BaCO3, SrCO3 and TiO2 powders during the conventional solid-state process, this coated structure is effective for the low temperature synthesis of barium strontium titanate (BST) powders. Sintering study of the powder synthesized by this method showed the dense and fine-grained BST ceramics could be obtained at a low sintering temperature. The grain size effect on the sensitivity of the dielectric constant vs. frequencies was discussed for the first time. Compared to the coarse-grained material, the fine-grained sample exhibits the better sensitivity of dielectric constant vs. frequency at ferroelectric state. It was shown that as the grain size decreased, the transition became diffuse, transition temperature (T-c) decreased and the dielectric constant increased.