Summary: This study attempts to gain mechanistic insight into the influence of ultrasound and cavitation in synthesis of polymethyl methacrylate (PMMA) and its nanocomposite using the organo-modified Cloisite 30B clay. A statistical design of experiments (Central Composite Design with Response Surface Methodology) has been used to find optimum values of quantities of monomer (15 g), initiator (0.75 g) and surfactant (0.87 g) in the synthesis of neat PMMA in 60 mL of total reaction volume. Using these optimized values, the nanocomposites have been synthesized using varying concentrations of clay. Characterization of the nanocomposites was done with standard techniques of XRD, FTIR, DSC, TGA, particle size analysis, zeta potential, SEM and TEM. The nanocomposites have been revealed to have better thermal properties (maximum degradation temperature of 277 C at T 5% and glass transition temperature of 120 C at 2% clay loading) as compared to neat PMMA. The results have been explained on the basis of basic physical and chemical principles of ultrasound and cavitation. The physical effect of ultrasound and cavitation of generation of intense microturbulence in the system has been revealed to play major role in the process rather than the chemical effect of generation of radicals.