Two blends between polyamide 6 (PA6) and Polyamide 6co6T (PA6co6T, a random copolymer between polyamide 6 and polyamide 6T) were fabricated by melt-mixing on a twin-screw extruder and the subsequent injection molding, or through the in-situ polymerization of F-caprolactam in the presence of PA6co6T. As far as the former method is concerned, there exist an obvious decline of toughness and a slight increase in strength and modulus; however, for the latter, there appear a remarkable improvement in toughness and a simultaneous moderate increase in strength and modulus. A series of characterizations were carried out including scanning electron microscopy, wide-angle X-ray diffraction, polarized optical microscopy, differential scanning calorimetry, dynamic mechanical analysis, and Fourier transform infrared spectrometry. It is found that both blends exhibit single glass transition on DMA tan delta curves. However, contrary to that of the melt-mixed blends, the glass transition temperature (T-g) of the in-situ ones decreases with increasing PA6co6T content. It is suggested that different mixing levels are the main reasons. Moreover, the addition of PA6co6T containing linear rigid segments conducts remarkable refinement of spherulites for the blends. Significantly different changes in the crystallographic form, spherulite size, crystalline content and perfection due to the introduction of PA6co6T for the two blends are ascribed to their varied thermomechanical histories and the presence of interchange reaction only for the in-situ blends. On the basis of the characterizations of the microstructures, the different trends of changes in the mechanical properties with the addition of PA6co6T for the two fabrication methods are discussed. (C) 2007 Wiley Periodicals, Inc.