We have successfully synthesized polymer (PMMA) nanoparticles which reflect precisely the size of monomer (MMA) droplets formed by the tandem acoustic emulsification. This new synthetic method involves sequential ultrasonic irradiation (20 kHz → 500 kHz → 1.6 MHz → 2.4 MHz) for acoustic emulsification of a water-insoluble MMA monomer in an aqueous medium, followed by emulsion photopolymerization in the obtained solution at an ambient temperature without using any surfactants. The present method is clearly a powerful tool for the synthesis of size-controlled polymer nanoparticles.