Polyester was melt-blended with three kinds of chain extenders 2,2-bis-oxazoline (Bis1), 1,3-phenylene-bis-2-oxazoline (Bis2), and polycarbodiimide (PCBI), which can react with carboxyl end group of polyester. The effects of chain extender content and mixing time on the carboxyl end group content, intrinsic viscosity and cold crystallization temperature of polyester were investigated. When less than 1.0 wt% of Bis1 was added, the carboxyl end group concentration decreased below 10 meq/kg. For the Bis2 concentration of less than 1.0 wt%, the carboxyl end group concentration was between 18~49 meq/kg. When more than 1.5 wt% of PCBI was added, the carboxyl end group concentration decreased below 10 meq/kg. The intrinsic viscosity increased and carboxyl end group concentration decreased with increasing PCBI content.