AbstractIn this work, the effects of adding small amounts of amorphous nylon (a‐nylon) on the properties of poly(butylene terephthalate)(PBT) have been studied. Miscibility and properties of PBT/a‐nylon blends were compared with those of PBT/nylon 6 blends. Blends were prepared in a twin‐screw extruder. The compositions of a‐nylon or nylon 6 were varied from 0 to 20 phr. Rheological, thermal, mechanical, and dynamic mechanical properties of the blends were examined using a capillary rheometer, a differential scanning calorimeter (DSC), a universal testing machine (UTM), and a Rheometrics Mechanical Spectrometer(RMS), respectively. The PBT showed a single endotherm around 225.4°C on the DSC thermogram at the first heating scan but exhibited a second endotherm at a temperature below that of the original endotherm at the second heating scan. When a‐nylon was added, the original endotherm was remarkably affected, but the second endotherm was not appreciably changed; i.e., the heat of fusion at the original endotherm decreased linearly with increasing a‐nylon content, meaning that the addition of a‐nylon affects significantly only the higher‐melting crystals of PBT. The impact strength of PBT was decreased with nylon 6 content but was increased with increasing content of a‐nylon. With an increase in the a‐nylon content, the tensile modulus and the flexural modulus of PBT were decreased. Of interest is, however, that the tensile strength was significantly increased with a‐nylon up to 15 phr. The melt viscosity of PBT was decreased with increasing a‐nylon content, whereas it was not significantly changed with nylon 6 addition; i.e. the addition of a‐nylon acted as a lubricant or a processing aid for PBT. The dynamic mechanical and the morphological studies showed that PBT was more miscible with a‐nylon than with nylon 6. It was also found that a‐nylon exhibited a compatibilizing effect for the PBT/nylon 6 blend of 70/30 wt% composition.