There are many methods to syntesize polystyrene latex. Emulsion polymerization is often used commercially, but initiator has the disadvantage for environmental pollution. The free radical is produced by ultrasound energy effect, polystyrene latex was synthesized using ultrasound energy instead of chemical initiator. As the ultrasonic irradiation time increased, average molecular weight increased and polydispersity decreased. And as the concentration of SDS increased, degree of polymerization increased and maximum value of degree of polymerization was shown when the concentration of SDS was 2 wt%, and reaction temperature was 40℃. During the polymerization, increase and decrease of molecular weight was repeated and molecular weight controlled monodisperse type polystyrene latex was synthesized by controlling of ultrasonic irradiation time and the concentration of SDS.