Reactive blending of polysulfone (PSU) and polyamide 6 (PA) was carried out at 20/80 weight ratio using a gram-scale mixer at 260 degrees C. Maleic anhydride-grafted PSU (PSU-MAH) and phthalic anhydride-terminated PSU (PSU-PhAH), having almost same molecular weight (M-w approximate to 20 k) and functional group content (ca. 90 mu mol/g) were prepared and used. The particle size reduction process was investigated by light scattering and transmission electron microscopy (TEM). Both reactive systems yielded finer particle size via faster particle size reduction process, compared with non-reactive system. Attainable particle size was in 10 nm-order in PSU-PhAH system, while in sub-mu m scale in PSU-MAH system. TEM observation at early stage of reactive blending in PSU-PhAH system showed that the in situ formed PSU-PA block copolymers escaped from interface to form micelle in PA matrix. Such micelle formation was observed only in the melt-mixed blend but not in a quiescently annealed blend, suggesting that the block copolymer prefer to locate at the interface under static condition but they are easily pulled out when PA-brushes are subjected to external shear forces during melt-mixing. The micelle formation leads to the fine dispersion of 10 nm level in the final blend. By contrast, the micelles were never observed in the PSU-MAH blend, in situ graft copolymer forming system, even at the late stages of the reactive processing. It suggests that the graft copolymer is highly resistant to be pulled out by the external forces. Then the graft copolymer seems to act as a simple emulsifier to attain the size reduction to sub-mu m level.