Barium titanate BaTiO3 is one of the most ferroelectrics. Its commonly used in capacitors due to its high dielectric constant. In this study, our goal is to determine the extent to which the change in the average ionic radius of A, B sites affects the structural, dielectric properties of this ceramic. In this study, barium titanate (Ba0.85X0.15TiO3, BaY0.15Ti0.85O3) (X = Pb, Ca, Co, Y= Zr, Si, Mn) was prepared by solid state reaction method in which the calcination temperature was 1200 °C for 2 hours. Structural characterization and dielectric properties were determined and calculated by using x-ray diffractometer, RCL-Meter (PM6303) at frequency 20 Hz using different temperature respectively. The results of the work showed that the average ionic radius at the A, B sites plays a large and clear role in determining the values of the dielectric constant and Curie temperature in perovskites in addition to the structural parameters.