Miniemulsion polymerizations of styrene and butyl acrylate in the presence of sodium dodecyl sulfate (SDS) as the surfactant and stearyl methacrylate (SMA) as reactive cosurfactant were carried out by using water-soluble (potassium persulfate) and oil-soluble (2,2-azobisisobutyronitrile) (AIBN) initiators respectively. Effects of the two initiators on the particle nucleation mechanisms are investigated. By comparison with the mean diameter of monomer droplets and polymer particles, it is shown that both homogeneous nucleation and monomer droplets nucleation coexist in the presence of a water-soluble initiator but homogeneous nucleation becomes less significant with SDS concentration increasing. Meanwhile, using oil-soluble initiator, the possibility of homogeneous nucleation is depressed effectively. As a result, monomer droplets are definitely the main loci of particle nucleation.