Structure-property relationships of segmented block copolyetheramides based on nylon 6 and polyoxypropylene (POP) were investigated as a function of the compositions and block lengths of the hard and soft segments by using differential scanning calorimetry, dynamic mechanical analysis, FT-IR spectroscopy, wide angle x-ray diffractometry (WAXD), and tensile testing. The nylon 6-D4 copolymers with the shortest soft segment exhibited the two-phase structure of a crystalline nylon 6 phase and an amorphous phase composed of the hard and soft segments. Also they showed the tensile properties characteristic of a plasticized thermoplastic. On the other hand, the nylon 6-D20 and nylon 6-D40 copolymers exhibited the three-phase structure of a crystalline nylon 6 phase, an amorphous nylon 6-rich phase, and an amorphous POP-rich phase. The nylon 6-D20 copolymers had better elastic properties with the increase of soft segment content; however, the nylon 6-D40 copolymers with the longest soft segment showed a substantial reduction of the elongation at break due to lack of the crosslinking density.