Submicron-size composite polymer particles consisting of two kinds of polymers were produced by seeded emulsion polymerization. Since most polymer pairs are not compatible, phase separation of the polymers proceeds throughout the polymerization and results in unique morphologies. Factors having a great influence on the formation of such morphology were clarified. Special properties can be obtained by controlling the morphology, as demonstrated using examples. Some anomalous composite particles were produced using this method.