In this study, the poly(methyl methacrylate)/polystyrene (PMMA/PS) composite polymer latex were synthesized by the method of soapless seeded emulsion polymerization. The morphology of the PMMA/PS composite polymer latex was core-shell structure with PMMA core and PS shell. There was an interpenetration layer between the PMMA core and PS shell. The interpenetration layer contained the PMMA/PS graft copolymer and physical interpenetration of PMMA and PS polymer chains. We found that the behavior of the interpenetration layer was the same as compatibilizer that would increase the degree of compatibility of polymer blend. In this investigation, three kinds of PMMA/PS polymer blends were prepared. The one was commercial PMMA and commercial PS polymer blend, weight fraction of PMMA/PS = 1/2 (polymer blend (A)). Another one was PMMA/PS core-shell composite polymer latex with PMMA core and PS shell, weight fraction of PMMA/PS = 1/2 (polymer blend (B)). The other one was commercial PMMA, PS and PMMA/PS core-shell composite latex blending (polymer blend (C)), i.e. polymer blend (B) blending with commercial PMMA and PS. The weight fraction of PMMA/PS of polymer blend (C) was 1/2, the weight percent of the commercial PMMA and PS was 70% and the core-shell composite latex was 30%. The morphology of polymer blend (A) was unstable. The effect of thermal annealing on the morphology of polymer blend (A) showed the phase separation structure. The thermal annealing would influence the tensile strength, Young＇s Modulus and viscosity properties of the polymer blend (A). The compatibility of the components of polymer blend (B) was very well. With increasing the thermal annealing time, the morphology of the polymer blend (B) appeared the interpenetration structure first, and then appeared the co-continuous structure. The morphology of polymer blend (C) was very stable. The effect of thermal annealing on the tensile strength, Young＇s Modulus and viscosity properties of the polymer blend (C) were insignificant.