Fairly uniform polystyrene-poly(methyl methacrylate) (PST-PMMA) composite microspheres were prepared by employing the SPG (Shirasu Porous Glass) membrane emulsification technique. PST, PMMA, and cosurfactant (lauryl alcohol, LOH) dissolved in dichloromethane (DCM) were used as a dispersed phase, and an aqueous phase containing poly(vinyl alcohol) and sodium lauryl sulfate was the continuous phase. The effects of LOH amount on the critical pressure of emulsification (P-cr), size distribution of droplets, and morphologies of final particles were investigated. Pt was found that P-cr decreased with increasing LOH amount because of preferential partition of LOH on the surface of the droplets in the initial stage of emulsification. When polymer concentration or PMMA/PST ratio was low, the size distribution of droplets decreased with increasing LOH amount, whereas an inverse trend was observed when both polymer concentration and PMMA/PST ratio were high. When polymer concentration was low, PST-PMMA core-shell particles always were obtained in the absence of LOH, irrespective of the PMMA/PST ratio. In the presence of LOH, however, microdomain, hemisphere, and inverted core-shell morphologies were formed as the PMMA/PST ratio decreased from 5/5 to 1/9 (g/g). When polymer concentration was high, different morphologies such as multiplet and inverted core-core-shell were observed. Theoretical calculations of morphologies were carried out, and agreement was obtained between experimental and calculated results.