We studied the morphology development of ternary immiscible blends through an interfacial reaction between components. This phenomenon was observed in two ternary blend systems; one is composed of polyamide(6) (PA6), polycarbonate (PC), and poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS), and the other is composed of PA6, PC, and polystyrene (PS), where PA6 forms the continuous matrix in both blend systems. Maleinated SEES (SEBS-gMA) or maleinated PS (PS-gMA) is incorporated with its unmodified polymer (un-SEBS and un-PS, respectively) at various ratios into the blends of PA6/PC. The blends of PA6/PC/un-SEBS and PA6/PC/un-PS show a similar phase formation in which the two dispersed polymers are stuck together in a PA6 matrix. The use of the maleinated polymers instead of their unmodified polymers in the blends of PA6/PC changes the phase formation drastically. The maleinated polymers react with amine end groups of PA6 at the interface during the melt mixing. Through this interfacial reaction, the domains of the maleinated polymers are dispersed in the PA6 matrix at about 100 nm in diameter, and at the same time the maleinated polymers encapsulate the PC domains. This means that the interfacial reaction induces the change of the formation of the domains composed of two dispersed phases. That is, the interfacial reaction changes the formation from "stack formation", where the two dispersed polymers are stuck together, to "capsule formation", where the PC domains are encapsulated by the other phase. Moreover, when both the unmodified and its maleinated polymers are incorporated together in a variety of ratios, the encapsulation by SEES onto the PC domains gradually becomes incomplete as the ratio of the unmodified SEES increases, whereas the encapsulation by PS of the PC is complete even when un-PS and PS-gMA are incorporated together. We discuss this morphology development in terms of the change of interfacial tensions between PA6 and maleinated polymers through the interfacial reaction. We assumed that the reduction of interfacial tension through the interfacial reaction is the driving force for this morphology development.