Nanoparticles of barium strontium titanate (BST) or Ba0.3Sr0.7TiO3 were synthesized by mechanical alloying and successive ultrasonic irradiation treatment. This research attempted to reduce the particles obtained through mechanical milling of mechanically alloyed powders to the particles with sizes in nanometer scale. The effectiveness of milling time on the synthesis of BST nanoparticles was confirmed by mean particle size evaluation which showed that a milling duration up to 60 h was required to bring down gradually the mean particle from 5.7 into 2.4 μm. The broadening lines of XRD pattern of the ultrasonically treated samples were analyzed using a high score plus (HSP) and a whole powder pattern modeling (WPPM) software to obtain the crystallite size distribution. The mean crystallite size of the treated particles was found 49 nm which is about 60 times lower than its mean particle size. It is shown that mechanical alloying through the sintering process followed by ultrasonic destruction will promote the formation of nanoparticles.