This paper describes the particle size characterization of mechanically alloyed Ba0.7Sr0.3TiO3 and Ba0.3Sr0.7TiO3 prepared with the aid of a high-power ultrasonic destruction. Analytical-grade BaCO3, TiO2 and SrCO3 with a purity greater than 99 wt.% were used as precursors for Ba0.7Sr0.3TiO3 and Ba0.3Sr0.7TiO3. The mechanically powders were respectively sintered at 1200 °C for 3 hours to form crystalline powders. This work is aimed at studying the effect of diameter ratio between reactor and transducer of a high power sonicator on the Ba0.7Sr0.3TiO3 and Ba0.3Sr0.7TiO3 nanoparticles formation. The presence of a single phase of the two materials was confirmed by X-Ray Diffraction (XRD). The concentration of the particles in demineralized water was 3.0 g/100 mL which become the object of 3 hours ultrasonic destruction subjected to the application of transducer in which the ratio between the diameter of the reactor and the transducer (D/d) was fixed at 1.4, 1.6 and 1.8 respectively. It was found that the mean particle size before the ultrasonic destruction was 538 nm for Ba0.7Sr0.3TiO3 and 480 nm for Ba0.3Sr0.7TiO3. With D/d of 1.8, the mean particle size of the two materials was found to decrease drastically to 38 nm and 24 nm, respectively. These mean particle sizes were respectively comparable with that of the crystallite size of the particles derived using the Whole Powder Pattern Modelling (WPPM) from which the mean crystallite size of 22 nm for Ba0.7Sr0.3TiO3 and 14 nm for Ba0.3Sr0.7TiO3 were obtained. It is then confirmed single nanocrystallite Ba0.7Sr0.3TiO3 and Ba0.3Sr0.7TiO3 particles were already achieved. We can conclude that the ultrasonic destruction to mechanically milled crystalline particles would be one of an effective way to produce nanoparticles.