To improve the thermal aging flexibility of poly(butylene terephthalate) (PBT), PBT was melt-blended with three type thermoplastic elastomer [poly ether-ester type (TPE1), polyester-ester type (TPE2), and poly(buthylene 2,6-naphthalate)/poly(tetramethylene glycol) block copolymer type (TPE3)], PBT/poly(ethylene terephthalate), (PET) alloy (Alloy), and phosphate type antioxidant (T1). The content of the three type TPEs and Alloy was fixed at 20 parts per 100 g of PBT. The morphology and thermal behavior of these blends have been investigated with scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and thermogravimetry (TG). In the case of PBT/Alloy-20 and PBT/TPE3-20 blends show clean fractured surface, whereas for PBT/TPE1-20 and PBT/TPE2-20 blends, the elongated pieces or fiber can be seen abundantly which indicates a good compatibility. TG traces show a significant shift of the weight loss toward higher temperature for PBT/Alloy-20, whereas PBT/TPE1-20, PBT/TPE2-20 and PBT/TPE3-20 blend decrease in thermal stability than PBT. To investigate the applicability for insulation material, the prepared blend samples were extruded an electric wire and flexibility and electric breakdown voltage (BDV) of wire after thermal aging were studied. For PBT/TPE1-20 and PBT/TPE2-20 blends did not show any cracks after flexibility test at 130 degrees C for 6 h and 225 degrees C for 30 min. In contrast PBT, PBT/Alloy-20, PBT/TPE3-20, and PBT/T1-1 showed a partial crack in the insulation after flexibility test at 130 degrees C for 6 h although its good flexibility at 225 degrees C for 30 min. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 3008-3015, 2009