The nano latex particles were prepared by the monomer starved semibatch emulsion polymerization with different mole ratio of styrene (St) to methyl methacrylate (MMA). The effects of monomer mole ratios on the polymerization kinetics, particle diameters and numbers were investigated. The relation between monomer feeding rate and the nucleation particle number was discussed. The nucleation mechanism was studied preliminarily. The results show that monomer conversion increases from 70% to 90% with increase of the MMA content; and the hydrophilic monomers of MMA would be beneficial to harvest the particles with the average volume diameter of 27.3 nm and unit volume (Np) of 8.5×1015 mL-1 at 12% solid content. The lgNp has a liner relation with lg monomer feeding rate (Ra), lgNp∝lgRa. Based on the results above, a conclusion is obtained that the nano latex particles could be formed by the monomer starved semibatch emulsion polymerization.