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) with various diameters of PSU particles : 90 nm by using phthalic anhydride-terminated PSU (PSU-PhAH), 0.2 mu m by carboxylic acidgrafted PSU (PSU-COOH), and 1.0 mu m by nonreactive PSU(nf-PSU). Isothermal crystallization was carried out at 200 degrees C. Light scattering studies showed that spherulites in the blends can grow to more than 10 mu m in diameter, as in the case of neat PA. It suggests that crystal growth in the radial direction can be achieved by circumventing the PSU obstacles; i.e., it does not matter how big or small they are in the range of 90 nm to 1 mu m The smaller the PSU particles, the slower was the overall rate of crystallization. The slower rate may be caused by the higher population of brush PA chains attached to PSU particles as block or graft chains, since the brushes may be hard to organize into crystal lamellae, compared with free chains. The blends showed rather broad wide-angle X-ray diffraction profiles. The smaller the PSU particles, the broader was the profile. The scattering from the particles themselves should appear only at extremely low angles (<0.1 degrees) and hardly affects the wide-angle X-ray profiles so that the results may imply that the presence of smaller PSU particles renders the less perfection of the PA crystal. This may be caused also by the higher population of brush chains. Note that the brush chains are formed only in the reactive systems and the population may be higher in the smaller particle systems. Even for the blend with smallest particle size, a small-angle X-ray scattering peak appeared at a magnitude of scattering vector of similar to 0.8 nm(-1), suggesting the evolution of interlamellar spacing of 8.7 nm. However, the scattering profile was much broader than that of neat PA. It implies a less ordered lamellar stacking in the blend.