An in-situ compatibilizer of poly(styrene-inn-glycidyl methacrylate) (PS-GMA) was used to study the effect of blend compositions on the morphology of two different blend systems, i.e. poly(ethylene-ran-acrylic acid) (PE-AA) and polystyrene (PS) blend, and poly(butylene terephthalate) (PET) and PS blend. The epoxy group in PS-GMA reacted easily with the carboxylic acid group in both PE-AA and PET, thus PS-graft-PE (Or PS-graft-PBT) copolymer as a compatibilizer was formed. We have shown that when there is no PS-GMA in both blend systems, the domain size of the dispersed phase in PE-AA/PS was larger than that in PBT/PS at the same blend composition. However, by increasing PS-GMA contents the reduction in the reduced domain size, defined by the ratio of the dispersed domain size with PS-GMA to the dispersed domain size without PS-GMA, in the PE-AA/PS blend system was greater than that in the PBT/PS blend system. These are attributed to the fact that Flory interaction parameter, chi, between PE-AA and PS was greater than that between PET and PS. Based upon the experimental observation that the reduced domain size with PS-GMA contents was collapsed into one master curve regardless of blend compositions in each blend system, a compatibilization mechanism of blend with a reactive compatibilizer was proposed and discussed in terms of the interfacial area occupied by one compatibilizer chain at the interface.