Poly(butylene terephalate) (PBT) and poly(butylene terephthalate-co-sebacate) (PBTS) copolymers containing 5 mol % and 10 mol % sebacate components (M-n = 12,700-14,600) were synthesized by polycondensation. The isothermal crystallization kinetics and melting behaviors after isothermal crystallization of the polymers were investigated by differential scanning calorimetry (DSC). The equilibrium melting temperatures of the polymers were determined by Hoffman-Weeks equation. Analysis of the crystallization kinetic data using the Avrami equation showed that the introduction of sebacate enhanced the crystallization of PBT in PBTS. And the Avrami exponent n varies in the range of 2.16-3.68, indicating that the isothermal crystallization follows two-and three-dimensional growth mechanism. The isothermal crystallization activation energies of the polymers were also calculated by the Arrhenius equation. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 735-742, 2011