Three-phase styrene-butyl acrylate composite polymer particles were synthesized by combining free radical polymerization, which yielded PS and PBA-co-PS, and controlled free radical polymerization, which yielded a large amount of PS-block-(PBA-co-PS)). Particle morphology evolves from core-shell when the particles did not contain block copolymer to hemispherical when a large amount of block copolymer was produced. The existing models could not predict this change in morphology. A general approach for the calculation of the equilibrium morphology of multiphase particles was developed using a Monte Carlo method.