By reactive blending of polyamide 6 (PA) with polysulfone (PSU) using a gram-scale mixer (Mini-Max Molder), we prepared a series of PA/PSU (80/20 wt. ratio) blends with various diameters of PSU particles; 70 nm by using phthalic anhydride-terminated PSU (PSU-PhAH), 0.4 mu m by epoxy-terminated PSU, 0.45 mu m by maleic anhydride-grafted PSU and 1.3 mu m by non-reactive PSU. By Light scattering (LS), small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), and differential scanning calorimetry, it was shown that there did not exist any difference in the crystalline morphology of PA matrix among the blends. Although difference in tensile strength among blends was small but on elongation it was large; the smaller PSU particles yielded larger elongation at break. The bulk-fracture toughness was shown to be higher for the blend with smaller PSU particles, Especially, PSU-PhAH blend showed a remarkably high toughness. In this blend, ductile fracture was shown by SEM observation. Transmission electron microscopic (TEM) observation confirmed the homogenous plastic deformation without interfacial debonding in the two-phase material. In contrast, other blends showed brittle fracture accompanied with interfacial debonding. The adhesive strength between PA and PSU-PhAH phases measured by asymmetric double cantilever beam method was shown to be much higher than in other systems. Thus, when the interfacial adhesive strength is high enough to provide adequate stress transfer, plastic deformation of brittle PSU particle can occur and hence the uniform plastic deformation of the whole material may render high toughness. The lower interfacial adhesion seems to yield in less massive plastic deformation to resulting in lower toughness.