A copolymer of styrene with acrylic acid (Poly(St–co–AA)) was directly synthesized by random emulsion copolymerization under ultrasonic irradiation without any volatile organic solvent (VOC) and emulsifier being used. Only water was used as solvent. The copolymer has been characterized by FT-IR spectra and H-NMR; its characteristic viscosity has been used to measure its average molecular weight. Our results show that high monomer conversion and short induction period were achieved. The effects of ultrasonic power and time on the average molecular weight and monomer conversion are discussed. The self-assembly behavior of the copolymer into regular pattern structures was imaged by atomic force microscopy (AFM).