Barium hexaferrite and strontium titanate are respectively well established permanent magnet and piezoelectric materials which are technologically and scientifically attractive due to their potential for various applications in the field of magnetic electronics functional materials. However, the material properties for both require a careful control of grain structure as well as microstructure design to meet specific applications. In this work, we report some results of materials characterization especially particles and crystallites in a BaFe12O19/SrTiO3 composite which were promoted during mechanical milling. The composite was synthesized using a planetary ball mill with a ball to powder ratio 10:1. Changing in the particle and crystallite-sizes at various milling time up to 60 hours are studied with the aid of particle-size analyzer and X-ray diffraction. It was found that the particle size of composite powders initially increased due to laminated layers formation of a composite and then decreased to an asymptotic value of ~8 μm as the milling time extended even to a relatively longer time. However, based on results of line broadening analysis the mean crystallite size of the particles was found in the nanometer scale. We thus believed that mechanical blending and milling of mixture components for the composite materials has promoted heterogeneous nucleation and only after successive sintering at 1100 °C the millled powder transformed into particles of nanograin. The crystallite growth kinetics at isothermal temperatures follow the relaxation equation with the activation energy value for BHF (QBHF) and STO (QSTO) are respectively 73.63 kJ/mol and 122.69 kJ/mol.