This work reports a new way to prepare monodisperse polystyrene (PS) particles by emulsifier-free miniemulsion polymerization. Only a small concentration (1.9 x 10(-3) g/mL) of potassium persulfate (KPS) is needed to initiate the polymerization of styrene (St) to produce a certain amount of PS oligomer, which can be utilized to form a stable miniemulsion. Subsequently, the St monomers polymerize by a droplet nucleation mechanism. The particle number keeps constant before and after polymerization.