Suspension-emulsion combined polymerization process, in which methyl methacrylate (MMA) emulsion polymerization constituents (EPC) were drop wise added to styrene (St) suspension polymerization system, was applied to prepare polystyrene/poly(methyl methacrylate) (PS/PMMA) composite particles. The influences of the feeding condition and the composition of EPC on the particle feature of the resulting composite polymer particles were investigated. It was found that PS/PMMA core-shell composite particles with a narrow particle size distribution and a great size would be formed when the EPC was added at the viscous energy dominated particle formation stage of St suspension polymerization with a suitable feeding rate, whereas St-MMA copolymer particles or PS/PMMA composite particles with imperfect core-shell structure would be formed when EPC was added at the earlier or later stage to St suspension polymerization, respectively. It was also showed that the EPC composition affected the composite particles formation process. The individual latex particles would exist in the final product when the concentrations of MMA monomer, sodium dodecyl sulfate emulsifier, and potassium persulfate initiator were great in the EPC. considering the feature of St suspension polymerization and morphology of PS/PMMA composite particles, the formation mechanism of PS/PMMA particles with core-shell structure was proposed. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 1659-1669,2009