We have successfully synthesized polymer nanoparticles which reflect precisely the size of monomer 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 monomer in an aqueous medium, followed by emulsion photopolymerization in the obtained solution at an ambient temperature without using any surfactants.