The linear composite polymer particles, which contain PMMA and PS components (PMMA/PS composite particle), were synthesized by the method of two stages soapless seeded emulsion polymerization. In this study, the aqueous medium of the emulsion system included poly(acrylic acid) (PAA). In the second stage reaction, the morphology of the PMMA/PS composite particles was changeable under the influence of PAA. In the initial period of second stage reaction, the morphology of the composite particle showed the core(PMMA)shell(PS) structure, in the middle period of second stage reaction, thermodynamic was the main factor to decide the morphology of the core and two shells, that is, PS as core, PMMA mixed with PS as inner shell, and PS as outer shell. Finally, the PMMA/PS composite particle showed the morphology of core(PS)-shell (PS phase dispersed in PMMA phase). This was in great contrast to the morphology of PMMA/PS composite particles, which were synthesized in the absence of PAA.