A group of heterogeneous latexes poly(butyl acrylate)/poly(styrene-co-methyl methacrylate) (PBA/P(St-co-MMA)) were prepared by a semi-continuous seeded emulsion polymerization process under monomer starved conditions. The glass transition temperature (Tg) and the mechanical properties of the film formed from the composite latex changed with the evolution of the particle morphology. A photon transmission method was used to monitor the phase structure evolution of films which were prepared from core-shell PBA/P(St-co-MMA) latex at room temperature and annealed at 383 K above Tg of the polymers. In addition, the changes of the surface of the film formed from the composite latex with time at 383 K were observed by AFM. The evidence illustrated that the film formed from the core-shell latex particles was metastable. The rearrangement of the phases could occur under proper conditions.