The effect of the amount of in situ formed graft copolymers on the blend morphology was investigated for immiscible polymer blends of poly(butylene terephthalate) (PBT) and polystyrene (PS) having various amounts of poly(styrene-ran-glycidyl methacrylate) (PS-GMA) as an in situ compatibilizer. Two different blending methods were used to prepare the blends: a melt blending (MB) and a solution blending followed by an oscillatory shearing at a molten state (SOM). The molecular weight of in situ PS-g-PBT copolymers formed from the reaction between PS-GMA and PET in the blend was determined by using high-temperature gel permeation chromatography (GPC). The concentration of in situ formed graft copolymers of PS-g-PBT in the blends [C-copolymer (blend)] prepared by either MB or SOM was determined by solvent extraction followed by Fourier transform infrared spectroscopy (FTIR) analysis. On the basis of the GPC and FTIR results, we concluded that the PS-g-PBT in the blends has 1.3-2 PET chains grafted onto a PS-GMA chain. From FTIR analysis and the morphology of the blends investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), we found that the interfacial areal density (Sigma) of in situ formed PS-g-PBT is similar to 0.1 chains/nm(2) for the blends prepared by SOM regardless of the amounts of PS-GMA added initially in the blend. However, although C-copolymer (blend) in the blend prepared by MB was similar to that prepared by SOM, the dispersed domain size of the former was larger than that of the latter, especially for blends with a large amount of PS-GMA (20 wt %). This suggests that at least some parts of in situ graft copolymers in the blend prepared by MB are located in the matrix phase as micelles with sizes of 20-50 mu m.