The isothermal crystallization kinetics and melting behaviors after isothermal crystallization of poly(butylene terephthalate) (PBT) and poly(butylene terephthalate-co-fumarate) (PBTF) containing 95/5, 90/10, and 80/20 molar ratios of terephthalic acid/fumaric acid were investigated by differential scanning calorimetry. The equilibrium melting temperatures of these polymers were estimated by HoffmanWeeks equation. So far as the crystallization kinetics was concerned, the Avrami equation was applied and the values of the exponent n for all these polymers are in the range of 2.502.96, indicating that the addition of fumarate does not affect the geometric dimension of PBT crystal growth. Crystallization activation energy (E) and nucleation constant (Kg) of PBTF copolymers are higher than that of PBT homopolymer, suggesting that the introduction of fumarate hinders the crystallization of PBT in PBTF. POLYM. ENG. SCI., 2013. (c) 2012 Society of Plastics Engineers