Polymer nanocomposites offer remarkable enhancement in properties compared to pristine polymers, and also compared to conventional micro- and macro-composites. The present study is focused on the preparation of nanocomposites by the usage of ultrasound. Polystyrene/alumina nanocomposites were synthesized using commercial alumina nanoparticles of approximately 200 nm size. These particles were dispersed in polystyrene solution by subjecting to ultrasound. Composites were synthesized at different filler loadings varying from 0.2% to 1% by weight. Dispersion characterization of these composites was carried out using scanning electron microscopy and X-ray diffraction. X-ray diffraction data revealed the incorporation of the filler into the polymer matrix. The mechanical properties, mainly tensile and impact properties, of the composites were studied to analyse the effect of nanofillers on the enhancement in functional properties compared to the neat polymer. Tensile strength and modulus varied with the filler loading in the polymer matrix. Theoretical prediction of the composite modulus was done using existing micromechanical models such as Halpin-Tsai and Mori-Tanaka. A comparison was made between the experimental moduli and the predicted values. Cavitation erosion studies were conducted on the composites and the mass eroded was quantified. © 2012 IMechE.