Nano-sized polystyrene (PS)-silica nanocomposite particles have been prepared by soap-free emulsion polymerization using a cationic initiator, 2,2'-azobis(isobutyramidine) hydrochloride (AIBA) with a colloidal silica (Ludox SM30, 7 nm diameter). The cationic initiator leads to the formation of the PS-silica nanocomposite particles by electrostatic interaction with negatively charged silica particles. Morphology, particle size distribution, reactivity and silica content of the particles were monitored on different reaction conditions such as pH, the addition time of silica sol and the amount of the silica sol. It is found that the nucleation of styrene monomer depends on the pH of water medium, the addition time of silica, the presence of silica in polymerization system. The reaction whose styrene monomer didn't react in water medium with pH 10 was progressed in the presence of silica sol to give nanocomposite particles. In the condition of constant pH 10 in the polymerization system, the increase of the amount of silica gave little influence to the changes in the particle size and particle size distribution of nanocomposite particles. The changes in the pH of medium gave much influence on the particle size and particle size distribution due to the changes in ionic interaction of silica and initiator. The silica content absorbed on the nanocomposite particles decreases with decreasing the pH values in the polymerization media.