Epoxycyclohexyl polyhedral oligomeric silsesquioxane (epoxy-POSS) was used to prepare a chain-extended poly(butylene terephthalate) (PBT) with a twin-screw extruder. The effect of epoxy-POSS on the melt flow index, mechanical properties, rheological behavior, and thermal properties of chain-extended PBT was investigated. PBT had an intrinsic viscosity of 1.1 dL/g and a carboxyl content of 21.6 equiv/10(6) g, but the PBT chain-extended with 2 wt % epoxy-POSS had an intrinsic viscosity of 1.7 dL/g and a carboxyl content lower than 7 equiv/10(6) g. After the addition of epoxy-POSS, the melt flow index of PBT dramatically decreased, the elongation at break increased greatly, the tensile strength increased slightly, and the thermal stability was also improved. (C) 2007 Wiley Periodicals, Inc.