The blends composed of polyamide 6 (PA6) and polyamide 66 (PA66) were obtained using two different preparation methods, one of which was the melt-mixing through a twin-screw extruder and the subsequent injection molding; and the other, the in situ blending through anionic polymerization of epsilon-caprolactam in the presence of PA66. For the former, there existed a remarkable improvement in toughness but a drastic drop in strength and modulus; however, for the latter, a reverse but less significant trend of mechanical properties change appeared. Various characterizations were conducted, including the analyses of crystalline morphology, crystallographic for-in, and crystallization and melting behaviors using polarized optical microscopy (POM), wide-angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC), respectively; observation of morphology of fractured surface with scanning electron microscope (SEM); measurement of glass transition through dynamic mechanical analysis (DMA); and the intermolecular interaction as well as the interchange reaction between the two components by Fourier transform infrared spectrometry (FT-IR) and C-13 solution NMR. The presence and absence of interchange reaction was verified for the in situ and melt-mixed blends, respectively. It is believed that the transreaction resulted in a drop in glass transition temperature (T,) for the in situ blends, contrary to an increase of T, with increasing PA66 content for the melt-mixed ones. And the two kinds of fabrication methods led to significant differences in the crystallographic form, spherulite size and crystalline content and perfection as well. Accordingly, it is attempted to explain the reasons for the opposite trends of changes in the mechanical properties for these two blends. (c) 2007 Wiley Periodicals, Inc.