The relation between the dispersed phase particle size and blend composition has been examined for blends of nylon 6 and styrene/acrylonitrile copolymer (SAN) over a wide range of compositions that traverse the phase inversion points for simple binary blends and those reactively compatibilized by the addition of imidized acrylic polymer (LA) or styrene/acrylonitrile/maleic anhydride terpolymer (SANMA). Nylon 6/SAN blends can be considered as a simpler version of industrially important polyamide (PA)/ABS blends; therefore, this study is useful for the better understanding of diminishing dispersed phase particles for PA/ABS blends with compatibilizers. For uncompatibilized blends, the relationship between particle size and composition is symmetric about the phase inversion composition; whereas, blends compatibilized with IA show an intense asymmetric behavior, i.e. SAN dispersed particles in a nylon 6 matrix are quite small, while nylon 6 particles in a SAN matrix are much larger and are elongated. On the other hand, the blends compatibilized with SANMA show a weak asymmetry. This asymmetry is seen for three nylon 6 materials of differing molecular weight. Possible causes for the asymmetric behavior have been considered. Concerning the conformation of graft polymer molecules at the interface, a crowding problem caused by the side chains of a graft polymer, when a nylon 6 phase disperses in a SAN matrix phase, may contribute to this asymmetric effect. The shift of the phase inversion composition to higher or lower nylon 6 volume fraction away from the 50/50 composition, may also directly contribute to the asymmetric trend, since generally the average domain size grows as the volume fraction of the minor phase increases due to the greater possibility of coalescence. Using Wu's equation, predicting the dispersed phase particle size, it is suggested that the viscosity increase of a nylon 6 phase due to the formation of graft polymers may affect the asymmetric behavior. However, the predicted asymmetry was less pronounced than the experimentally observed asymmetry.