We have shown that dimpled polystyrene particles with a number of pores on the surface can be produced via surfactant-free emulsion polymerization of styrene with an oil-soluble initiator in the aqueous medium. Styrene monomer droplets containing an oil-soluble initiator (AIBN) were dispersed in water under ultrasound irradiation in surfactant-free conditions, and then styrene droplets were polymerized in the ultrasound field. Bubbles in the oil droplet generated by ultrasound irradiation led to pore formation in polymer particles.